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The Quantum Explanation of Spectral Lines

2011-02-16T01:15:00.000-08:00

The explanation for exact spectral lines for each substance was provided by the quantum theory. In his 1913 model of the hydrogen atom Niels Bohr showed that the observed series of lines could be explained by assuming that electrons are restricted to atomic orbits in which their orbital angular momentum is an integral multiple of the quantity h/2π, where h is Planck's constant. The integer multiple (e.g., 1, 2, 3 …) of h/2π is usually called the quantum number and represented by the symbol n.

When an electron changes from an orbit of higher energy (higher angular momentum) to one of lower energy, a photon of light energy is emitted whose frequency ν is related to the energy difference ΔE by the equation ν=ΔE/h.For hydrogen, the frequencies of the spectral lines are given by ν=cR (1/n_f²−1/n_i²) where c is the speed of light, Ris the Rydberg constant, and n_f and n_i are the final and initial quantum numbers of the electron orbits (n_i is always greater than n_f). The series of spectral lines for which n_f=1 is known as the Lyman series; that for n_f=2 is the Balmer series; that for n_f=3 is the Paschen series; that for n_f=4 is the Brackett series; and that for n_f=5 is the Pfund series. The Bohr theory was not as successful in explaining the spectra of other substances, but later developments of the quantum theory showed that all aspects of atomic and molecular spectra can be explained quantitatively in terms of energy transitions between different allowed quantum states.

spectrum

2011-02-16T01:05:00.000-08:00

Spectrum, arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph). Physicists often find it useful to separate a beam of particles into a spectrum according to their energy.

Continuous and Line Spectra

Dispersion, the separation of visible light into a spectrum, may be accomplished by means of a prism or a diffraction grating. Each different wavelength or frequency of visible light corresponds to a different color, so that the spectrum appears as a band of colors ranging from violet at the short-wavelength (high-frequency) end of the spectrum through indigo, blue, green, yellow, and orange, to red at the long-wavelength (low-frequency) end of the spectrum. In addition to visible light, other types of electromagnetic radiation may be spread into a spectrum according to frequency or wavelength.

The spectrum formed from white light contains all colors, or frequencies, and is known as a continuous spectrum. Continuous spectra are produced by all incandescent solids and liquids and by gases under high pressure. A gas under low pressure does not produce a continuous spectrum but instead produces a line spectrum, i.e., one composed of individual lines at specific frequencies characteristic of the gas, rather than a continuous band of all frequencies. If the gas is made incandescent by heat or an electric discharge, the resulting spectrum is a bright-line, or emission, spectrum, consisting of a series of bright lines against a dark background. A dark-line, or absorption, spectrum is the reverse of a bright-line spectrum; it is produced when white light containing all frequencies passes through a gas not hot enough to be incandescent. It consists of a series of dark lines superimposed on a continuous spectrum, each line corresponding to a frequency where a bright line would appear if the gas were incandescent. The Fraunhofer lines appearing in the spectrum of the sun are an example of a dark-line spectrum; they are caused by the absorption of certain frequencies of light by the cooler, outer layers of the solar atmosphere. Line spectra of either type are useful in chemical analysis, since they reveal the presence of particular elements. The instrument used for studying line spectra is the spectroscope.

learn more: The Quantum Explanation of Spectral Lines

conservation laws

2011-02-15T01:36:00.000-08:00

Conservation laws, in physics, basic laws that together determine which processes can or cannot occur in nature; each law maintains that the total value of the quantity governed by that law, e.g., mass or energy, remains unchanged during physical processes. Conservation laws have the broadest possible application of all laws in physics and are thus considered by many scientists to be the most fundamental laws in nature.

Conservation of Classical Processes

Most conservation laws are exact, or absolute, i.e., they apply to all possible processes; a few conservation laws are only partial, holding for some types of processes but not for others. By the beginning of the 20th cent. physics had established conservation laws governing the following quantities: energy, mass (or matter), linear momentum, angular momentum, and electric charge. When the theory of relativity showed (1905) that mass was a form of energy, the two laws governing these quantities were combined into a single law conserving the total of mass and energy.

Conservation of Elementary Particle Properties

With the rapid development of the physics of elementary particles during the 1950s, new conservation laws were discovered that have meaning only on this subatomic level. Laws relating to the creation or annihilation of particles belonging to the baryon and lepton classes of particles have been put forward. According to these conservation laws, particles of a given group cannot be created or destroyed except in pairs, where one of the pair is an ordinary particle and the other is an antiparticle belonging to the same group. Recent work has raised the possibility that the proton, which is a type of baryon, may in fact be unstable and decay into lighter products; the postulated methods of decay would violate the conservation of baryon number. To date, however, no such decay has been observed, and it has been determined that the proton has a lifetime of at least 10³¹ years. Two partial conservation laws, governing the quantities known as strangeness and isotopic spin, have been discovered for elementary particles. Strangeness is conserved during the so-called strong interactions and the electromagnetic interactions, but not during the weak interactions associated with particle decay; isotopic spin is conserved only during the strong interactions.

Conservation of Natural Symmetries

One very important discovery has been the link between conservation laws and basic symmetries in nature. For example, empty space possesses the symmetries that it is the same at every location (homogeneity) and in every direction (isotropy); these symmetries in turn lead to the invariance principles that the laws of physics should be the same regardless of changes of position or of orientation in space. The first invariance principle implies the law of conservation of linear momentum, while the second implies conservation of angular momentum. The symmetry known as the homogeneity of time leads to the invariance principle that the laws of physics remain the same at all times, which in turn implies the law of conservation of energy. The symmetries and invariance principles underlying the other conservation laws are more complex, and some are not yet understood.

Three special conservation laws have been defined with respect to symmetries and invariance principles associated with inversion or reversal of space, time, and charge. Space inversion yields a mirror-image world where the "handedness" of particles and processes is reversed; the conserved quantity corresponding to this symmetry is called space parity, or simply parity, P. Similarly, the symmetries leading to invariance with respect to time reversal and charge conjugation (changing particles into their antiparticles) result in conservation of time parity, T, and charge parity, C. Although these three conservation laws do not hold individually for all possible processes, the combination of all three is thought to be an absolute conservation law, known as the CPT theorem, according to which if a given process occurs, then a corresponding process must also be possible in which particles are replaced by their antiparticles, the handedness of each particle is reversed, and the process proceeds in the opposite direction in time. Thus, conservation laws provide one of the keys to our understanding of the universe and its material basis.

momentum

2011-02-15T01:31:00.000-08:00

Momentum, in mechanics, the quantity of motion of a body, specifically the product of the mass of the body and its velocity. Momentum is a vector quantity; i.e., it has both a magnitude and a direction, the direction being the same as that of the velocity vector. When an external force acts upon a body or a system of bodies in motion, it causes a change in the momentum of the body. The impulse of a force acting on a body is the product of the force and the duration of time in which it acts and is equal to the change in momentum of the body. When no external force acts upon a body in motion or a system of bodies there is no change in the total momentum even though, as in the case of a system of bodies, there may be an internal disturbance of the system resulting in changes in the momenta of individual bodies. This conclusion is commonly known as the principle of the conservation of momentum (see conservation laws, in physics). The momentum of a body should not be confused with its kinetic energy. The distinction between them can be seen in the action of a pile driver. The distance to which the pile is driven depends upon its kinetic energy; the length of time required for the action to cease, upon its momentum. In addition to the momentum a body has because of its linear motion, the body may also have angular momentum because of rotation. The angular momentum of a particle rotating about a point is equal to the product of the mass of the particle, its angular velocity, and the square of its distance from the axis of rotation. More simply, the angular momentum is the product of the instantaneous linear momentum and the distance. Angular momentum is a vector quantity directed perpendicular to the plane of motion.

Official Symbols and Names for the Elements

2011-02-14T06:42:00.000-08:00

Each element is assigned an official symbol by the International Union of Pure and Applied Chemistry (IUPAC). For example, the symbol for carbon is C, and the symbol for silver is Ag [Lat. argentum = silver]. There are several ways of designating an isotope. One designation consists of the name or symbol of the element followed by a hyphen and the mass number of the isotope; thus the isotope of carbon with mass number 12 can be designated carbon-12 or C-12. The mass number is often written as a superscript, e.g., C¹²; sometimes the atomic number is written as a subscript preceding the symbol, e.g., ₆C¹². The IUPAC rules for nomenclature of inorganic chemistry state that the subscript atomic number and superscript mass number should both precede the symbol, e.g., ^_¹²₆C.

Many isotopes were given special names and symbols when they were first discovered in natural radioactive decay series (e.g., uranium-235 was called actinouranium and represented by the symbol AcU). This practice is discouraged in the modern nomenclature except in the case of hydrogen. The isotopes hydrogen-2 and hydrogen-3 are usually called deuterium and tritium, respectively. Hydrogen-1, the most abundant isotope, has the name protium but is usually simply called hydrogen. Newly discovered elements that have been synthesized by one laboratory and not yet confirmed by a second are given a provisional name based on Greek and Latin roots; when the discovery is confirmed, the laboratory that first made it may suggest a name for the element.

nonmetal

2011-02-14T06:33:00.000-08:00

Nonmetal, chemical element possessing certain properties by which it is distinguished from a metal. In general, this distinction is drawn on the basis that a nonmetal tends to accept electrons and form negative ions and that its oxide is acidic. Nonmetals are poor conductors of heat and electricity (see conduction) and do not have the luster of metals. Arsenic, antimony, selenium, and tellurium exhibit both nonmetallic and metallic properties and are called metalloids. Unlike the metals, which are all solids (with the exception of mercury) under ordinary conditions of temperature and pressure, the nonmetals appear in all three states. Argon, chlorine, fluorine, helium, hydrogen, krypton, neon, nitrogen, oxygen, and xenon are normally gases. Bromine is a liquid. Boron, carbon, iodine, phosphorus, silicon, and sulfur are solids. Certain of them, e.g., boron, carbon, iodine, silicon, and sulfur, form crystals, as do the metals. In hardness they vary considerably. Carbon in its allotropic form, the diamond, is the hardest element known. With the exception of carbon, sulfur, nitrogen, oxygen, and the inert gases—argon, helium, krypton, neon, and xenon—the nonmetals do not occur uncombined in nature, but exist in numerous relatively abundant compounds, among which are the oxides, halides (binary halogen compounds), sulfides, carbonates, nitrates, phosphates, silicates, and sulfates. With a few exceptions, the nonmetallic elements are important chiefly for their compounds. For the properties and uses of specific nonmetals, see the separate articles on these elements.

Properties of the Elements

2011-02-14T06:29:00.000-08:00

Properties of an element are sometimes classed as either chemical or physical. Chemical properties are usually observed in the course of a chemical reaction, while physical properties are observed by examining a sample of the pure element. The chemical properties of an element are due to the distribution of electrons around the atom's nucleus, particularly the outer, or valence, electrons; it is these electrons that are involved in chemical reactions. A chemical reaction does not affect the atomic nucleus; the atomic number therefore remains unchanged in a chemical reaction.

Some properties of an element can be observed only in a collection of atoms or molecules of the element. These properties include color, density, melting point, boiling point, and thermal and electrical conductivity. While some of these properties are due chiefly to the electronic structure of the element, others are more closely related to properties of the nucleus, e.g., mass number.

The elements are sometimes grouped according to their properties. One major classification of the elements is as metals, nonmetals, and metalloids. Elements with very similar chemical properties are often referred to as families; some families of elements include the halogens, the inert gases, and the alkali metals. In the periodic table the elements are arranged in order of increasing atomic weight in such a way that the elements in any column have similar properties.

compound

2011-02-14T06:15:00.000-08:00

Compound, in chemistry, a substance composed of atoms of two or more elements in chemical combination, occurring in a fixed, definite proportion and arranged in a fixed, definite structure. A compound is often represented by its chemical formula. The formula for water is H₂O, and for sodium chloride, NaCl. The formula weight of a compound can be determined from its formula. The molecular weight of a molecular compound can be determined from its molecular formula. Two or more distinct compounds that have the same molecular formula but different properties are called isomers.

Formation and Decomposition of Compounds

Compounds are formed from simpler substances by chemical reaction. Some compounds can be formed directly from their constituent elements, e.g., water from hydrogen and oxygen: 2H₂ + O₂ → 2H₂O. Other compounds are formed by reaction of an element with another compound; e.g., sodium hydroxide (NaOH) is formed (and hydrogen gas released) by the reaction of sodium metal with water: 2Na + 2H₂O → 2NaOH + H₂↑. Compounds are also made by reaction of other compounds; e.g., sodium hydroxide reacts with hydrogen chloride (HCl) to form sodium chloride and water: HCl + NaOH → NaCl + H₂O. Complex molecules such as proteins are formed by a series of reactions involving elements and simple compounds.

Compounds can be decomposed by chemical means into elements or simpler compounds. Water is broken down into hydrogen and oxygen by electrolysis. Candle wax, a mixture of hydrocarbons, is changed in the candle flame by combustion (with oxygen) to a mixture of the simpler compounds carbon dioxide (CO₂) and water. Life is based on numerous reactions in which energy is stored and released as compounds are produced and decomposed.

Properties of Compounds

A compound has unique properties that are distinct from the properties of its elemental constituents. One familiar chemical compound is water, a liquid that is nonflammable and does not support combustion. It is composed of two elements: hydrogen, an extremely flammable gas, and oxygen, a gas that supports combustion. A compound differs from a mixture in that the components of a mixture retain their own properties and may be present in many different proportions. The components of a mixture are not chemically combined; they can be separated by physical means. A mixture of hydrogen and oxygen gases is still a gas and can be separated by physical methods. If the mixture is ignited, however, the two gases undergo a rapid chemical combination to form water. Although the hydrogen and oxygen can occur in any proportion in a mixture of gases, they are always combined in the exact proportion of two atoms of hydrogen to one atom of oxygen when combined in the compound water. Another familiar compound is sodium chloride (common salt). It is composed of the silvery metal sodium and the greenish poisonous gas chlorine combined in the proportion of one atom of sodium to one atom of chlorine.

Molecular and Ionic Compounds

Water is a molecular compound; it is made up of electrically neutral molecules, each containing a fixed number of atoms. Sodium chloride is an ionic compound; it is made up of electrically charged ions that are present in fixed proportions and are arranged in a regular, geometric pattern (called crystalline structure) but are not grouped into molecules. The atoms in a compound are held together by chemical bonding.

synthetic elements / transactinide elements

2011-02-14T05:45:00.000-08:00

Synthetic elements, in chemistry, radioactive elements that were not discovered occurring in nature but as artificially produced isotopes. They are technetium (at. no. 43), which was the first element to be synthesized, promethium (at. no. 61), astatine (at. no. 85), francium (at. no. 87), and the transuranium elements (at. no. 93 and beyond in the periodic table). Some of these elements have since been shown to exist in minute amounts in nature, usually as short-lived members of natural radioactive decay series (see radioactivity).

The synthetic elements through at. no. 100 (fermium) are created by bombarding a heavy element, such as uranium or plutonium, with neutrons or alpha particles. The synthesis of the transfermium elements (elements with at. no. 101 or greater) is accomplished by the fusion of the nuclei of two lighter elements. Elements 101 through 106 were first produced by fusing the nuclei of slightly lighter elements, such as californium, with those of light elements, such as carbon. Elements 107 through 112 were first produced by fusing the nuclei of medium-weight elements, such as bismuth or lead, with those of other medium-weight elements, such as iron, nickel, or zinc. Element 114 was first produced by fusing the nuclei of plutonium and calcium and subsequently by fusing the nuclei of lead and krypton, as was element 116. Element 115 was produced by bombarding americium with calcium, and element 113 resulted from the radioactive decay of element 115. The claim by Lawrence Berkeley National Laboratory to have created element 118 has been retracted.)

The transfermium elements are produced in very small quantities (one atom at a time), and identification is therefore very difficult because of half-lives ranging from minutes to milliseconds and the need to identify the products by methods other than known chemical separations. This has led to controversy over reported discoveries and over the naming of the elements. It has been predicted that one isotope of element 114—containing 114 protons and 184 neutrons—would be very stable because its nucleus would have a full complement of protons and neutrons. Termed an "island of stability," its half-life might be measured in years. However, none of the three isotopes of element 114 synthesized as yet have as many as 184 neutrons, and their half-lives are still in the millisecond range.

Transactinide elements (chemistry), in the periodic table, elements with atomic numbers higher than 103.

ununhexium

2011-02-14T05:33:00.000-08:00

Ununhexium, artificially produced radioactive chemical element; symbol Uuh; at. no. 116; mass number of most stable isotope 292; m.p., b.p., sp. gr., and valence unknown. Situated in Group 16 of the periodic table, it is expected to have properties similar to those of polonium and tellurium.

In 1999 a research team at the Lawrence Berkeley National Laboratory in Calif. bombarded lead-208 atoms with high-energy krypton-86 ions to create, apparently, ununoctium (element 118) atoms. The Uuo-293 isotope that they synthesized emitted an alpha particle to decay into Uuh-289, which has a life-life of about 0.6 millisecond, which then emitted an alpha particle to decay into ununquadium (element 114). Although the Berkeley laboratory retracted its claim for creating ununoctium in 2001, other research teams have since created ununhexium directly. No name has yet been adopted for element 116, which is therefore called ununhexium, from the Latin roots un for one and hex for six, under a convention for neutral temporary names proposed by the International Union of Pure and Applied Chemistry (IUPAC) in 1980.

ununoctium

2011-02-14T05:21:00.000-08:00

Ununoctium (y'nənŏk`tēəm), artificially produced radioactive chemical element; symbol Uuo; at. no. 118. Scientists from the Joint Institute for Nuclear Research in Dubna, Russia, and Lawrence Livermore National Laboratory in California collaborated in the discovery of ununoctium in experiments conducted in 2002 and 2005. They bombarded atoms of californium-249 with ions of calcium-48. Among the products of the bombardments were three atoms of ununoctium-294 (one atom in 2002 and two in 2005), each of which decayed in 0.9 milliseconds into an atom of ununhexium by emitting an alpha particle. No name has yet been adopted for element 118, which is therefore called ununoctium, from the Latin roots un for one and oct for eight, under a convention for neutral temporary names proposed by the International Union of Pure and Applied Chemistry (IUPAC) in 1980.

In 1999 a research team at the Lawrence Berkeley National Laboratory in Calif. bombarded lead-208 atoms with high-energy krypton-86 ions to create what an analysis showed to be three atoms of element 118 with mass number 293 and a half-life of less than a millisecond. In 2001, however, the team retracted its claim to have produced ununoctium after other laboratories failed to reproduce their results and after a reanalysis of the original data did not show the production of element 118. A subsequent investigation suggested that the original finding was the result of fraud on the part of one of the team scientists.

See also synthetic elements; transactinide elements; transuranium elements.

Elements

2011-02-14T05:07:00.000-08:00

Element	Symbol	Atomic Number	Atomic Weight¹	Melting Point (Degrees Celsius)	Boiling Point (Degrees Celsius)
actinium	Ac	89	227.0278	1050.	3200. ±300
aluminum	Al	13	26.98154	660.37	2467.
americium	Am	95	(243)	1172.	2600.
antimony	Sb	51	121.75	630.74	1750.
argon	Ar	18	39.948	−189.2	−185.7
arsenic	As	33	74.9216	817. (at 28 atmospheres)	613. (sublimates)
astatine	At	85	(210)	302. (est.)	337. (est.)

barium	Ba	56	137.33	725.	1640.
berkelium	Bk	97	(247)	1050.	2590.
beryllium	Be	4	9.01218	1278. ±5	2970.
bismuth	Bi	83	208.9804	271.3	1560. ±5
bohrium	Bh	107	(262)	—	—
boron	B	5	10.81	2300.	2550. (sublimates)
bromine	Br	35	79.904	−7.2	58.78

cadmium	Cd	48	112.41	320.9	765.
calcium	Ca	20	40.08	839. ±2	1484.
californium	Cf	98	(251)	900.	1470.
carbon	C	6	12.011	∼3550.	4827.
cerium	Ce	58	140.12	799.	3426.
cesium	Cs	55	132.9054	28.40	669.3
chlorine	Cl	17	35.453	−100.98	−34.6
chromium	Cr	24	51.996	1857. ±20	2672.
cobalt	Co	27	58.9332	1495.	2870.
copper	Cu	29	63.546	1083.4 ±0.2	2567.
curium	Cm	96	(247)	1340. ±40	3110.

darmstadtium	Ds	110	(271)	—	—
dubnium	Db	105	(262)	—	—
dysprosium	Dy	66	162.50	1412.	2562.

einsteinium	Es	99	(252)	857.	—
erbium	Er	68	167.26	1529.	2863.
europium	Eu	63	151.96	822.	1597.

fermium	Fm	100	(257)	1527.	—
fluorine	F	9	18.998403	−219.62	−188.14
francium	Fr	87	(223)	(27) (est.)	(677) (est.)

gadolinium	Gd	64	157.25	1313. ±1	3266.
gallium	Ga	31	69.72	29.78	2403.
germanium	Ge	32	72.59	937.4	2830.
gold	Au	79	196.9665	1064.43	2808.

hafnium	Hf	72	178.49	2227. ±20	4602.
hassium	Hs	108	(265)	—	—
helium	He	2	4.0026	<−272.2	−268.934
holmium	Ho	67	164.9304	1474.	2425.
hydrogen	H	1	1.00794	−259.14	−252.87

indium	In	49	114.82	156.61	2080.
iodine	I	53	126.9045	113.5	184.35
iridium	Ir	77	192.22	2410.	4130.
iron	Fe	26	55.847	1535.	2750.

krypton	Kr	36	83.80	−156.6	−152.30 ±0.10

lanthanum	La	57	138.9055	921.	3457.
lawrencium	Lr	103	(262)	1627.	—
lead	Pb	82	207.2	327.502	1740.
lithium	Li	3	6.941	180.54	1342.
lutetium	Lu	71	174.967	1663.	3395.

magnesium	Mg	12	24.305	648.8 ±0.5	1090.
manganese	Mn	25	54.9380	1244. ±3	1962.
meitnerium	Mt	109	(266)	—	—
mendelevium	Md	101	(258)	827.	—
mercury	Hg	80	200.59	−38.842	356.58
molybdenum	Mo	42	95.94	2617.	4612.

neodymium	Nd	60	144.24	1021.	3068.
neon	Ne	10	20.179	−248.67	−246.048
neptunium	Np	93	237.0482	640. ±1	3902. (est.)
nickel	Ni	28	58.69	1453.	2732.
niobium	Nb	41	92.9064	2468. ±10	4742.
nitrogen	N	7	14.0067	−209.86	−195.8
nobelium	No	102	(259)	827.	—

osmium	Os	76	190.2	3045. ±30	5027. ±100
oxygen	O	8	15.9994	−218.4	−182.962

palladium	Pd	46	106.42	1554.	2970.
phosphorus	P	15	30.97376	44.1 (white)	280. (white)
platinum	Pt	78	195.08	1772.	3827. ±100
plutonium	Pu	94	(244)	641.	3232.
polonium	Po	84	(209)	254.	962.
potassium	K	19	39.0983	63.25	760.
praseodymium	Pr	59	140.9077	931.	3512.
promethium	Pm	61	(145)	1042	3000. (est.)
protactinium	Pa	91	231.0359	<1600.	4026.

radium	Ra	88	226.0254	700.	1140.
radon	Rn	86	(222)	−71.	−61.8
rhenium	Re	75	186.207	3180.	5627. (est.)
rhodium	Rh	45	102.9055	1966. ±3	3727. ±100
roentgenium	Rg	111	(272)	—	—
rubidium	Rb	37	85.4678	38.89	686.
ruthenium	Ru	44	101.07	2310.	3900.
rutherfordium	Rf	104	(261)	—	—

samarium	Sm	62	150.36	1072. ±5	1791.
scandium	Sc	21	44.9559	1541.	2831.
seaborgium	Sg	106	(266)	—	—
selenium	Se	34	78.96	217.	684.9 ±1.0
silicon	Si	14	28.0855	1410.	2355.
silver	Ag	47	107.8682	961.93	2212.
sodium	Na	11	22.98977	97.81 ±0.03	882.9
strontium	Sr	38	87.62	269.	1384.
sulfur	S	16	32.06	112.8	444.674

tantalum	Ta	73	180.9479	2996.	5425. ±100
technetium	Tc	43	(98)	2200.	4877.
tellurium	Te	52	127.60	449.5 ±0.3	989.8 ±3.8
terbium	Tb	65	158.9254	1356.	3123.
thallium	Tl	81	204.383	303.5	1457. ±10
thorium	Th	90	232.0381	1750.	∼4790.
thulium	Tm	69	168.9342	1545. ±15	1947.
tin	Sn	50	118.69	231.9681	2270.
titanium	Ti	22	47.88	1660. ±10	3287.
tungsten	W	74	183.85	3410. ±20	5660.

ununbium	Uub	112	(285)	—	—
ununhexium	Uuh	116	(292)	—	—
ununoctium	Uuo	118	(294)	—	—
ununpentium	Uup	115	(288)	—	—
ununquadium	Uuq	114	(289)	—	—
ununtrium	Uut	113	(284)	—	—
uranium	U	92	238.0289	1132.3 ±0.8	3818.

vanadium	V	23	50.9415	1890. ±10	3380.

xenon	Xe	54	131.29	−111.9	−107.1 ±3
ytterbium	Yb	70	173.04	819.	1194.
yttrium	Y	39	88.9059	1522. ±8	3338.

zinc	Zn	30	65.38	419.58	907.
zirconium	Zr	40	91.22	1852. ±2	4377.

¹ Parentheses indicate most stable isotope.

element

2011-02-14T04:54:00.000-08:00

Element, in chemistry, a substance that cannot be decomposed into simpler substances by chemical means. A substance such as a compound can be decomposed into its constituent elements by means of a chemical reaction, but no further simplification can be achieved. An element can, however, be decomposed into simpler substances, such as protons and neutrons or various combinations of them, by the methods of particle physics, e.g., by bombardment of the nucleus.

The Atom

The smallest unit of a chemical element that has the properties of that element is called an atom. Many elements (e.g., helium) occur as single atoms. Other elements occur as molecules made up of more than one atom. Elements that ordinarily occur as diatomic molecules include hydrogen, nitrogen, oxygen, and the halogens, but oxygen also occurs as a triatomic form called ozone. Phosphorus usually occurs as a tetratomic molecule, and crystalline sulfur occurs as molecules containing eight atoms.

Atomic Number and Mass Number

Regardless of how many atoms the element is composed of, each atom has the same number of protons in its nucleus, and this is different from the number in the nucleus of any other element. Thus this number, called the atomic number (at. no.), defines the element. For example, the element carbon consists of atoms all with at. no. 6, i.e., all having 6 protons in the nucleus; any atom with at. no. 6 is a carbon atom. By 2006, 117 elements were known, ranging from hydrogen with an at. no. of 1 to an as yet unnamed element (temporarily known as ununoctium) with an at. no. of 118. (See the table entitled Elements for an alphabetical list of all the elements, including their symbols, atomic numbers, atomic weights, and melting and boiling points.) The nuclei of most atoms also contain neutrons. The total number of protons and neutrons in the nucleus of an atom is called the mass number. For example, the mass number of a carbon atom with 6 protons and 6 neutrons in its nucleus is 12.

Isotopes

Although all atoms of an element have the same number of protons in their nuclei, they may not all have the same number of neutrons. Atoms of an element with the same mass number make up an isotope of the element. All known elements have isotopes; some have more than others. Hydrogen, for example, has only 3 isotopes, while xenon has 16. Approximately 300 naturally occurring isotopes are known, and more than 2,500 radioactive isotopes have been artificially produced (see synthetic elements). There are 13 isotopes of carbon, having from 2 to 14 neutrons in the nucleus and therefore mass numbers from 8 to 20.

Not all of the elements have stable isotopes. Some have only radioactive isotopes, which decay to form other isotopes, usually of other elements (see radioactivity). In some cases all the isotopes of an element are very unstable, and the element is therefore not found in nature. Only 94 of the elements are known to occur naturally on earth. Of these, 6 occur in minute amounts produced by the decay of other elements. These 6 extremely scarce elements and those that do not occur at all naturally were discovered when they were produced in the laboratory; they are often called the man-made, artificially produced, or synthetic elements.

Atomic Mass and Atomic Weight

Atoms are not very massive; a carbon atom weighs about 2 × 10−23 grams. Because atoms have so little mass, a unit much smaller than the gram is used. In the current system (adopted in 1960–61) the unit of atomic mass, called atomic mass unit (amu), is defined as exactly 1-12 the mass of an atom of carbon-12. The atomic weight of an element is the mean (weighted average) of the atomic masses of all the naturally occurring isotopes. Carbon has two principal naturally occurring isotopes, carbon-12 and carbon-13. Carbon-12, whose mass is defined as exactly 12 amu, constitutes 98.89% of naturally occurring carbon; carbon-13, whose mass is 13.00335 amu, constitutes 1.11%. (There are also small traces of the radioactive isotope carbon-14.) The atomic weight of the element is determined by multiplying the percent abundance of each isotope by the atomic mass of the isotope, adding these products, and dividing by 100. However, isotope abundance is often determined by the medium of the source, solid, liquid, or gas, and the average atomic weight may fluctuate. Thus, for carbon, [(98.89 × 12.000) + (1.11 × 13.00335)]/100 = 12.01115, which is the atomic weight of the element carbon in amu. Certain synthetic elements exist only momentarily in the form of a few short-lived isotopes; in such cases the concept of atomic weight cannot be applied.

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mass number

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Mass number, often represented by the symbol A, the total number of nucleons (neutrons and protons) in the nucleus of an atom. All atoms of a chemical element have the same atomic number (number of protons in the nucleus) but may have different mass numbers (from having different numbers of neutrons in the nucleus). Atoms of an element with the same mass number make up an isotope of the element. Different isotopes of the same element cannot have the same mass number, but isotopes of different elements often do have the same mass number, e.g., carbon-14 (6 protons and 8 neutrons) and nitrogen-14 (7 protons and 7 neutrons).

atomic mass unit

2011-02-14T04:45:00.000-08:00

Atomic mass unit or amu, in chemistry and physics, unit defined as exactly 1-12 the mass of an atom of carbon-12, the isotope of carbon with six protons and six neutrons in its nucleus. One amu is equal to approximately 1.66 × 10⁻²⁴ grams.

atomic mass

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Atomic mass, the mass of a single atom, usually expressed in atomic mass units (amu). Most of the mass of an atom is concentrated in the protons and neutrons contained in the nucleus. Each proton or neutron weighs about 1 amu, and thus the atomic mass is always very close to the mass number (total number of protons and neutrons in the nucleus). Atoms of an isotope of an element all have the same atomic mass. Atomic masses are usually determined by mass spectrography (see mass spectrograph). They have been determined with great relative accuracy, but their absolute value is less certain.

atomic weight

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Atomic weight, mean (weighted average) of the masses of all the naturally occurring isotopes of a chemical element, as contrasted with atomic mass, which is the mass of any individual isotope. Although the first atomic weights were calculated at the beginning of the 19th cent., it was not until the discovery of isotopes by F. Soddy (c.1913) that the atomic mass of many individual isotopes was determined, leading eventually to the adoption of the atomic mass unit as the standard unit of atomic weight.

Effect of Isotopes in Calculating Atomic Weight

Most naturally occurring elements have one principal isotope and only insignificant amounts of other isotopes. Therefore, since the atomic mass of any isotope is very nearly a whole number, most atomic weights are nearly whole numbers, e.g., hydrogen has atomic weight 1.00797 and nitrogen has atomic weight 14.007. However, some elements have more than one principal isotope, and the atomic weight for such an element—since it is a weighted average—is not close to a whole number; e.g., the two principal isotopes of chlorine have atomic masses very nearly 35 and 37 and occur in the approximate ratio 3 to 1, so the atomic weight of chlorine is about 35.5. Some other common elements whose atomic weights are not nearly whole numbers are antimony, barium, boron, bromine, cadmium, copper, germanium, lead, magnesium, mercury, nickel, strontium, tin, and zinc.

Atomic weights were formerly determined directly by chemical means; now a mass spectrograph is usually employed. The atomic mass and relative abundance of the isotopes of an element can be measured very accurately and with relative ease by this method, whereas chemical determination of the atomic weight of an element requires a careful and precise quantitative analysis of as many of its compounds as possible.

Development of the Concept of Atomic Weight

J. L. Proust formulated (1797) what is now known as the law of definite proportions, which states that the proportions by weight of the elements forming any given compound are definite and invariable. John Dalton proposed (c.1810) an atomic theory in which all atoms of an element have exactly the same weight. He made many measurements of the combining weights of the elements in various compounds. By postulating that simple compounds always contain one atom of each element present, he assigned relative atomic weights to many elements, assigning a weight of 1 to hydrogen as the basis of his scale. He thought that water had the formula HO, and since he found by experiment that 8 weights of oxygen combine with 1 weight of hydrogen, he assigned an atomic weight of 8 to oxygen. Dalton also formulated the law of multiple proportions, which states that when two elements combine in more than one proportion by weight to form two or more distinct compounds, their weight proportions in those compounds are related to one another in simple ratios. Dalton's work sparked an interest in determining atomic weights, even though some of his results—such as that for oxygen—were soon shown to be incorrect.

While Dalton was working on weight relationships in compounds, J. L. Gay-Lussac was experimenting with the chemical reactions of gases, and he found that, when under the same conditions of temperature and pressure, gases react in simple whole-number ratios by volume. Avogadro proposed (1811) a theory of gases that holds that equal volumes of two gases at the same temperature and pressure contain the same number of particles, and that these basic particles are not always single atoms. This theory was rejected by Dalton and many other chemists.

P. L. Dulong and A. T. Petit discovered (1819) a specific-heat method for determining the approximate atomic weight of elements. Among the first chemists to work out a systematic group of atomic weights (c.1830) was J. J. Berzelius, who was influenced in his choice of formulas for compounds by the method of Dulong and Petit. He attributed the formula H₂O to water and determined an atomic weight of 16 for oxygen. J. S. Stas later refined many of Berzelius's weights. Stanislao Cannizzaro applied Avogadro's theories to reconcile atomic weights used by organic and inorganic chemists.

The availability of fairly accurate atomic weights and the search for some relationship between atomic weight and chemical properties led to J. A. R. Newlands's table of "atomic numbers" (1865), in which he noted that if the elements were arranged in order of increasing atomic weight "the eighth element, starting from a given one, is a kind of repetition of the first." He called this the law of octaves. Such investigations led to the statement of the periodic law, which was discovered independently (1869) by D. I. Mendeleev in Russia and J. L. Meyer in Germany. T. W. Richards did important work on atomic weights (after 1883) and revised some of Stas's values.

isotope

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Isotope, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F. Soddy in explaining aspects of radioactivity; the first stable isotope (of neon) was discovered by J. J. Thomson. The nuclei of isotopes contain identical numbers of protons, equal to the atomic number of the atom, and thus represent the same chemical element, but do not have the same number of neutrons. Thus isotopes of a given element have identical chemical properties but slightly different physical properties and very different half-lives, if they are radioactive (see half-life). For most elements, both stable and radioactive isotopes are known. Radioactive isotopes of many common elements, such as carbon and phosphorus, are used as tracers in medical, biological, and industrial research. Their radioactive nature makes it possible to follow the substances in their paths through a plant or animal body and through many chemical and mechanical processes; thus a more exact knowledge of the processes under investigation can be obtained. The very slow and regular transmutations of certain radioactive substances, notably carbon-14, make them useful as "nuclear clocks" for dating archaeological and geological samples. By taking advantage of the slight differences in their physical properties, the isotopes may be separated. The mass spectrograph uses the slight difference in mass to separate different isotopes of the same element. Depending on their nuclear properties, the isotopes thus separated have important applications in nuclear energy. For example, the highly fissionable isotope uranium-235 must be separated from the more plentiful isotope uranium-238 before it can be used in a nuclear reactor or atomic bomb.

mass spectrograph

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Mass spectrograph, device used to separate electrically charged particles according to their masses; a form of the instrument known as a mass spectrometer is often used to measure the masses of isotopes of elements. J. J. Thomson and F. W. Aston showed (c.1900) that magnetic and electric fields can be used to deflect streams of charged particles traveling in a vacuum, and that the degree of bending depends on the masses and electric charges of the particles. In the mass spectrograph the particles, in the form of ions, pass through deflecting fields (produced by carefully designed magnetic pole pieces and electrodes) and are detected by photographic plates. The beam of ions first passes through a velocity selector, consisting of a combination of electric and magnetic fields that eliminates all particles except those of a given velocity. The remaining ion beam then enters an evacuated chamber where a magnetic field bends it into a semicircular path ending at the photographic plate. The radius of this path depends upon the mass of the particles (all other factors, such as velocity and charge, being equal). Thus, if in the original stream isotopes of various masses are present, the position of the blackened spots on the plate makes possible a calculation of the isotope masses. The mass spectrograph is widely used in chemical analysis and in the detection of impurities.

prism

2011-02-14T02:36:00.000-08:00

Prism, in optics, a piece of translucent glass or crystal used to form a spectrum of light separated according to colors. Its cross section is usually triangular. The light becomes separated because different wavelengths or frequencies are refracted (bent) by different amounts as they enter the prism obliquely and again as they leave it (see refraction). The shorter wavelengths, toward the blue or violet end of the spectrum, are refracted by the greatest amount; the longer wavelengths, toward the red end, are refracted the least. The Nicol prism is a special type of prism made of calcite; it is used for polarization of light.

Antioxidant

2010-10-15T05:23:00.000-07:00

Antioxidant, type of molecule that neutralizes harmful compounds called free radicals that damage living cells, spoil food, and degrade materials such as rubber, gasoline, and lubricating oils. Antioxidants can take the form of enzymes in the body, vitamin supplements, or industrial additives. They are routinely added to metals, oils, foodstuffs, and other materials to prevent free radical damage.

Free radicals are produced under certain environmental conditions and during normal cellular function in the body. These molecules are missing an electron, giving them an electric charge. To neutralize this charge, free radicals try to steal an electron from, or donate an electron to, a neighboring molecule. This process, called oxidation, creates a new free radical from the neighboring molecule. The newly created free radical, in turn, searches out another molecule and steals or donates an electron, setting off a chain reaction that can damage hundreds of molecules.

Antioxidants halt this chain reaction. Some antioxidants are themselves free radicals, donating electrons to stabilize and neutralize the dangerous free radicals. Other antioxidants work against the molecules that form free radicals, destroying them before they can begin the domino effect that leads to oxidative damage.

Learn more: Antioxidants in the Human Body; Dietary Sources of Antioxidants; Antioxidants in Industry

Toxin

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Toxin, poisonous substance produced by the metabolic activities of certain living organisms, including bacteria, insects, plants, and reptiles.

Some bacteria secrete toxins in tissues that they colonize; these are true toxins. Other bacteria retain most of the poisonous material within themselves, and the toxins are liberated only when the bacteria become disintegrated by chemical, physical, or mechanical means. In addition to bacterial toxins, the characteristic poisons and venoms produced by various plants are called phytotoxins, and those produced by animals are called zootoxins. The more important true toxins causing infection in humans are those of botulism, dysentery, tetanus, and diphtheria. Because of their extreme susceptibility to various chemical and physical influences, such as light, heat, and age, toxins are difficult to isolate, and knowledge of toxins has been gained through the lesions and symptoms that they produce when injected into animals.

Although all toxins are poisonous, in order to become effective they must chemically combine with the animal cells. With the exception of botulin, they are destroyed by the gastrointestinal juices. Although the exact chemical nature of toxins is unknown, they are generally thought to be toxalbumins, substances closely allied to proteins. It has also been abundantly demonstrated that toxins are colloid in nature and bear a close resemblance to enzymes. Toxins are absolutely specific synthetic products, unlike ptomaines, which are cleavage products from the medium on which the bacteria grow. In certain forms, toxins can give rise to antibodies, natural defensive substances produced in the body. Toxoids are toxins that are treated to destroy their toxicity but that remain potent enough to create antibodies when injected into the body.

Serotonin

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Serotonin, neurotransmitter, or chemical that transmits messages across the synapses, or gaps, between adjacent cells. Among its many functions, serotonin is released from blood cells called platelets to activate blood vessel constriction and blood clotting. In the gastrointestinal tract, serotonin inhibits gastric acid production and stimulates muscle contraction in the intestinal wall. Its functions in the central nervous system and effects on human behavior—including mood, memory, and appetite control—have been the subject of a great deal of research. This intensive study of serotonin has revealed important knowledge about the serotonin-related cause and treatment of many illnesses.

Serotonin is produced in the brain from the amino acid tryptophan, which is derived from foods high in protein, such as meat and dairy products. Tryptophan is transported to the brain, where it is broken down by enzymes to produce serotonin. In the process of neurotransmission, serotonin is transferred from one nerve cell, or neuron, to another, triggering an electrical impulse that stimulates or inhibits cell activity as needed. Serotonin is then reabsorbed by the first neuron, in a process known as reuptake, where it is recycled and used again or converted into an inactive chemical form and excreted.

Casein

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Casein, group of proteins precipitated when milk is mildly acidified. Casein constitutes about 80% of the total proteins in cow's milk and about 3% of its weight. It is the chief ingredient in cheese. When dried, it is a white, amorphous powder without taste or odor. Casein dissolves slightly in water, extensively in alkalies or strong acids.

Casein is used as a food supplement and as an adhesive, a constituent of water paints, and a finishing material for paper and textiles. A variety of casein, known by the modified name paracasein, is preferred for making a plastic, through the reaction of the casein with formaldehyde, that goes into the manufacture of buttons and other small objects. It is produced by adding the enzyme rennin to milk, forming a precipitate different from the material precipitated by acids.

Calorie

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Calorie, metric unit of heat measurement. The small, or gram, calorie (cal) is usually specified in science and engineering as the amount of heat required to raise the temperature of 1 g of water from 14.5° to 15.5° C. The temperature interval is sometimes specified in other ways. The definition now generally accepted and standard in thermochemistry, is that 1 cal equals 4.1840 joules (J).

A slightly different calorie is used in engineering, the international calorie, which equals 1/860 international watt-hour (W h). A large calorie, or kilocalorie (Cal), usually referred to as a calorie and sometimes as a kilogram calorie, equals 1000 cal and is the unit used to express the energy-producing value of food in the calculation of diets.

Antioxidants in Industry

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Antioxidants are also used in industry as product additives and in food processing and preservation. Industrial antioxidants slow or prevent oxidative damage that causes food to spoil, rubber to harden, fats and oil to change color or go rancid, and gasoline to oxidize. Foods that are commonly preserved with antioxidant additives include cheese, bread, and oil. Antioxidants used as food preservatives include vitamin C and the synthetic antioxidants butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA). These antioxidants are added to foods in concentrations of much less than 1 percent.

Dietary Sources of Antioxidants

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Vitamin C, also known as ascorbic acid, is a well known antioxidant that may prevent cataracts and cancers of the stomach, throat, mouth, and pancreas. It may also prevent the oxidation of LDL cholesterol, lowering the risk of heart disease. Foods that are high in vitamin C include strawberries, oranges, broccoli, and brussels sprouts.

Beta-carotene absorbs free radicals that target molecules in the cell membrane. Studies suggest that in addition to reducing the risk of cataract, cancer, and heart attack, beta-carotene may also reduce the risk of stroke. Beta-carotene occurs naturally in orange-colored fruits and vegetables and dark green, leafy vegetables. Some of the best sources of beta-carotene are sweet potatoes, spinach, and carrots.

As an antioxidant, vitamin E may also protect from heart disease and cataract and may strengthen the immune system. Good sources of vitamin E include wheat germ oil and sunflower seeds.

Carcinogen

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Carcinogen, any chemical, biological, or physical agent that can potentially be a cause of cancer. The term is most commonly applied to chemicals introduced into the environment by human activity. Researchers label a substance a carcinogen if it causes a statistically significant increase in some form of neoplasm, or anomalous cell growth, when applied to a population of previously unexposed organisms. The modes of cancer initiation are still little understood, however, and efforts to establish the carcinogenic hazards of substances have aroused great controversy. The question of the usefulness of laboratory tests on animals in assessing human risks is particularly complex. The more recent development of short-term tests using cell cultures of microorganisms, however, is considered a major advance in carcinogen research.

Substances indicted as carcinogenic over the past few decades include the pesticides DDT, Kepone, and EDB; the synthetic hormone DES; the artificial sweetener cyclamate; asbestos; and a wide range of other industrial and environmental substances.

Antioxidants in the Human Body

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About 5 percent of the oxygen humans breathe is converted into free radicals. The presence of free radicals in the body is not always detrimental. Free radicals produced in normal cellular metabolism are vital to certain body functions, such as fighting disease or injury. When tissue is diseased or damaged, the body’s immune system sends disease fighting cells to the site, where they produce free radicals in an effort to destroy foreign invaders.

But as the body ages or is subjected to environmental pollutants, such as cigarette smoke, overexposure to sunlight, or smog, the body becomes overwhelmed by free radicals. An excessive number of free radicals causes damage by taking electrons from key cellular components of the body, such as protein, lipids, and deoxyribonucleic acid (DNA), the molecule that carries genetic information in every living cell. These reactions make cells more vulnerable to cancer-causing chemicals, called carcinogens. Free radicals may lead to heart disease by oxidizing low-density lipoprotein (LDL) cholesterol, the so-called bad cholesterol. Researchers now believe that only the oxidized form of LDL cholesterol leads to hardening of the arteries, a condition that can ultimately lead to heart disease. Free radicals have also been implicated in cataract, a clouding of the lens of the eye that can lead to blindness.

Sulfuric Acid

2009-04-16T00:28:00.000-07:00

Sulfuric Acid, corrosive, oily, colorless liquid, with a specific gravity of 1.85. It melts at 10.36° C (50.6° F), boils at 340° C (644° F), and is soluble in all proportions in water. When sulfuric acid is mixed with water, considerable heat is released. Unless the mixture is well stirred, the added water may be heated beyond its boiling point and the sudden formation of steam may blow the acid out of its container (see Acids and Bases). The concentrated acid destroys skin and flesh, and can cause blindness if it gets into the eyes. The best treatment is to flush away the acid with large amounts of water. Despite the dangers created by careless handling, sulfuric acid has been commercially important for many years. The early alchemists prepared it in large quantities by heating naturally occurring sulfates to a high temperature and dissolving in water the sulfur trioxide thus formed. About the 15th century a method was developed for obtaining the acid by distilling hydrated ferrous sulfate, or iron vitriol, with sand. In 1740 the acid was produced successfully on a commercial scale by burning sulfur and potassium nitrate in a ladle suspended in a large glass globe partially filled with water.

Sulfuric acid is a strong acid, that is, in aqueous solution it is largely changed to hydrogen ions (H+) and sulfate ions. Each molecule gives two H+ ions, thus sulfuric acid is dibasic. Dilute solutions of sulfuric acid show all the behavior characteristics of acids. They taste sour, conduct electricity, neutralize alkalies, and corrode active metals with formation of hydrogen gas. From sulfuric acid one can prepare both normal salts containing the sulfate group and acid salts containing the hydrogen sulfate group.

Smog

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Smog, mixture of solid and liquid fog and smoke particles formed when humidity is high and the air so calm that smoke and fumes accumulate near their source. Smog reduces natural visibility and often irritates the eyes and respiratory tract. In dense urban areas, the death rate usually goes up considerably during prolonged periods of smog, particularly when a process of heat inversion creates a smog-trapping ceiling over a city.

Smog prevention requires control of smoke from furnaces; reduction of fumes from metal-working and other industrial plants; and control of noxious emissions from automobiles, trucks, and incinerators. In the U.S. internal-combustion engines are regarded as the largest contributors to the smog problem, emitting large amounts of contaminants, including unburned hydrocarbons and oxides of nitrogen. The number of undesirable components in smog, however, is considerable, and the proportions highly variable. They include ozone, sulfur dioxide, hydrogen cyanide, and hydrocarbons and their products formed by partial oxidation. Fuel obtained from fractionation of coal and petroleum produces sulfur dioxide, which is oxidized by atmospheric oxygen, forming sulfur trioxide. Sulfur trioxide is in turn hydrated by the water vapor in the atmosphere to form sulfuric acid.

The so-called photochemical smog, which irritates sensitive membranes and damages plants, is formed when nitrogen oxides in the atmosphere undergo reactions with the hydrocarbons energized by ultraviolet and other radiations from the sun. See Air Pollution.

Camphor

2009-01-18T03:01:00.008-08:00

Camphor, volatile, white, crystalline compound,with a characteristic aromatic odor. Ordinary camphor is obtained from the camphor tree, Cinnamomum camphora, which grows in Asia and Brazil. The camphor is distilled by steaming chips of the root, stem, or bark. The leaves of certain plants, such as tansy and feverfew, contain a second form of camphor, which is not used commercially. A racemic form is present in the oil of an Asian chrysanthemum and is also produced synthetically for most commercial uses. Camphor is used in the manufacture of celluloid and explosives and medicinally in liniments and other preparations for its mild antiseptic and anesthetic qualities. It is poisonous if ingested in large amounts.

Camphor is insoluble in water, soluble in organic solvents, and melts at 176° C (349° F) and boils at 209° C (405° F).

Ozone

2009-01-18T03:01:00.007-08:00

Ozone (Greek ozein, “to smell”), pale blue, highly poisonous gas with a strong odor. Ozone is considered a pollutant at ground level, but the ozone layer of the upper atmosphere protects life on Earth from the Sun’s harmful ultraviolet radiation.

Ozone is one of three forms, called allotropes, of the element oxygen. Ozone is triatomic, meaning that it has three atoms in each molecule (formula O3). Ordinary, or diatomic, oxygen (O2) is more stable than ozone and accounts for the bulk of oxygen in the atmosphere. Electrical sparks and ultraviolet light can cause ordinary oxygen to form ozone. The presence of ozone sometimes causes a detectable odor near electrical outlets.

PROPERTIES

At normal temperatures and pressures ozone is a gas with a specific gravity of 2.144 (about 1.5 times the density of ordinary oxygen gas). Ozone accounts for only a tiny fraction of the atmosphere and is normally invisible, but high concentrations of ozone gas are pale blue. The gas condenses to a liquid at -111.9°C (-169.52°F) and freezes at -192.5°C (-314.5°F). Liquid ozone is deep blue, and is diamagnetic (repelled by magnetic fields). Solid ozone is dark purple. Ozone is much more active chemically than ordinary oxygen. It is used in purifying water, sterilizing air, and bleaching certain foods.

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ENVIRONMENTAL EFFECTS

Chlorofluorocarbons (CFCs)

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Chlorofluorocarbons (CFCs), family of synthetic chemicals that are compounds of the elements chlorine, fluorine, and carbon. CFCs are stable, nonflammable, noncorrosive, relatively nontoxic chemicals and are easy and inexpensive to produce. During the 1970s, scientists linked CFCs to the destruction of Earth’s ozone layer. The manufacture of CFCs has since been banned in most countries.

USES

Scientists developed the first CFCs during the late 1920s. The compounds subsequently became used in a wide range of industrial products in the United States, Europe, and Japan. Manufacturers used CFCs as refrigerants in refrigerators, freezers, air conditioners, and heat pumps, and as propellants in aerosols and medical inhalers. CFCs also served as insulating foams in packaging materials, furniture, bedding, and car seats. Cleaning agents for electronic circuit boards, metal parts, and dry cleaning processes also used CFCs.

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Allotrope

2009-01-18T03:01:00.003-08:00

Allotrope, two or more distinct physical forms of a chemical element in the same physical state. The term allotropy comes from the Greek allos tropos meaning “another shape.” Allotropes arise because of differing arrangements of an element’s atoms within its molecules or crystals.

One of the best-known examples of allotropy is carbon, which has multiple distinct allotropes including graphite, diamond, and buckminsterfullerene. Carbon atoms in diamond form a rigid, three-dimensional structure, with each carbon atom bonded to four other carbon atoms. In graphite the carbon atoms form stacks of flat honeycomb layers with only weak intermolecular forces between layers, while buckminsterfullerene forms balls and tubes with structures reminiscent of the geodesic domes designed by the architect Richard Buckminster Fuller.

Elements exhibiting allotropy include arsenic, antimony, iron, oxygen, phosphorus, selenium, sulfur, and tin.

There are two main kinds of allotropy, monotropy and enantiotropy. Monotropy (monotropic allotropy) occurs when one form of a substance is stable at all temperatures, while any other forms are metastable, and change (sometimes very slowly) into the stable form. For carbon, graphite is the stable monotrope, while diamond and buckminsterfullerene change, extremely slowly, into graphite. Phosphorus also exhibits monotropy: Red phosphorus is stable, while white (sometimes called yellow) phosphorus is metastable.

Enantiotropy (enantiotropic allotropy) occurs when one solid form of the substance changes into another solid form of the same substance when at a definite transition temperature. Tin, which changes from white tin to gray tin below 13°C (55°F), is an example of this type of allotropy. Gray tin is much more brittle than white tin. The Greek philosopher Aristotle recorded tin statues collapsing in the intense cold as long ago as the 4th century BC.

Infrared Radiation

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Infrared Radiation, emission of energy as electromagnetic waves in the portion of the spectrum just beyond the limit of the red portion of visible radiation (see Electromagnetic Radiation). The wavelengths of infrared radiation are shorter than those of radio waves and longer than those of light waves. They range between approximately 10-6 and 10-3 (about 0.0004 and 0.04 in). Infrared radiation may be detected as heat, and instruments such as bolometers are used to detect it.

Infrared radiation is used to obtain pictures of distant objects obscured by atmospheric haze, because visible light is scattered by haze but infrared radiation is not. The detection of infrared radiation is used by astronomers to observe stars and nebulas that are invisible in ordinary light or that emit radiation in the infrared portion of the spectrum.

An opaque filter that admits only infrared radiation is used for very precise infrared photographs, but an ordinary orange or light-red filter, which will absorb blue and violet light, is usually sufficient for most infrared pictures. Developed about 1880, infrared photography has today become an important diagnostic tool in medical science as well as in agriculture and industry. Use of infrared techniques reveals pathogenic conditions that are not visible to the eye or recorded on X-ray plates. Remote sensing by means of aerial and orbital infrared photography has been used to monitor crop conditions and insect and disease damage to large agricultural areas, and to locate mineral deposits. In industry, infrared spectroscopy forms an increasingly important part of metal and alloy research, and infrared photography is used to monitor the quality of products.

Fossil Fuels

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Fossil Fuels, energy-rich substances that have formed from long-buried plants and microorganisms. Fossil fuels, which include petroleum, coal, and natural gas, provide most of the energy that powers modern industrial society. The gasoline that fuels our cars, the coal that powers many electrical plants, and the natural gas that heats our homes are all fossil fuels.

Chemically, fossil fuels consist largely of hydrocarbons, which are compounds composed of hydrogen and carbon. Some fossil fuels also contain smaller amounts of other compounds. Hydrocarbons form from ancient living organisms that were buried under layers of sediment millions of years ago. As accumulating sediment layers exerted increasing heat and pressure, the remains of the organisms gradually transformed into hydrocarbons. The most commonly used fossil fuels are petroleum, coal, and natural gas. These substances are extracted from the earth’s crust and, if necessary, refined into suitable fuel products, such as gasoline, heating oil, and kerosene. Some of these hydrocarbons may also be processed into plastics, chemicals, lubricants, and other nonfuel products. Geologists have identified other types of hydrocarbon-rich deposits that can serve as fuels. Such deposits, which include oil shale, tar sands, and gas hydrates, are not widely used because they are too costly to extract and refine.

The majority of fossil fuels are used in the transportation, manufacturing, residential heating, and electric-power generation industries. Crude petroleum is refined into gasoline, diesel fuel, and jet fuel, which power the world’s transportation system. Coal is the fuel most commonly burned to generate electric power, and natural gas is used primarily in commercial and residential buildings for heating water and air, for air conditioning, and as fuel for stoves and other heating appliances.

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Formation of Fossil Fuels

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Fossil fuels formed from ancient organisms that died and were buried under layers of accumulating sediment. As additional sediment layers built up over these organic deposits, the material was subjected to increasing temperatures and pressures. Over millions of years, these physical conditions chemically transformed the organic material into hydrocarbons.

Most organic debris is destroyed at the earth's surface by oxidation or by consumption by microorganisms. Organic material that survives to become buried under sediments or deposited in other oxygen-poor environments begins a series of chemical and biological transformations that may ultimately result in petroleum, natural gas, or coal. Many such deposits occur in sedimentary basins (depressed areas in the earth’s crust where sediments accumulate), and along continental shelves. Sediments may accumulate to depths of several thousand feet in a basin, exerting pressures up to one hundred million pascals (tens of thousands of pounds per square inch) and temperatures of several hundred degrees on the organic material. Over millions of years, these conditions can chemically transform the organic material into petroleum, natural gas, coal, or other types of fossil fuels.

A. Petroleum Formation
B. Coal Formation
C. Natural Gas Formation
D. Other Fossil Fuels

Petroleum Formation

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Petroleum formed chiefly from ancient, microscopic plants and bacteria that lived in the ocean and saltwater seas. When these microorganisms died and settled to the seafloor, they mixed with sand and silt to form organic-rich mud. As layers of sediment accumulated over this organic ooze, the mud was gradually heated and slowly compressed into shale or mudstone, chemically transforming the organic material into petroleum and natural gas.

Sometimes, the petroleum and natural gas would slowly fill the tiny holes within nearby porous rocks, which geologists call reservoir rocks. Because these porous rocks were usually filled with water, the liquid and gaseous hydrocarbons (which are less dense and lighter than water) migrated upward, through the earth’s crust, sometimes for long distances. A portion of these hydrocarbons would eventually encounter an impermeable (nonporous) layer of rock in an anticline, salt dome, fault trap, or stratigraphic trap. The impermeable rock would trap the hydrocarbons, creating a reservoir of petroleum and natural gas. Exploration geologists seek these underground formations because they often contain recoverable petroleum deposits. The fluids and gases caught in these geologic traps typically separate into three layers: water (highest density, bottom layer), petroleum (middle layer), and natural gas (low density, top layer).

Coal Formation

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Coal is a solid fossil fuel formed from ancient plants—including trees, ferns, and mosses—that grew in swamps and bogs or along coastal shorelines. Generations of these plants died and were gradually buried under layers of sediment. As the sedimentary overburden increased, the organic material was subjected to increasing heat and pressure that caused the organic material to undergo a number of transitional states to form coal. The mounting pressure and temperature caused the original organic material, which was rich in carbon, hydrogen, and oxygen, to become increasingly carbon-rich and hydrogen- and oxygen-poor. The successive stages of coal formation are peat (partially carbonized plant matter), lignite (soft brownish-black coal with low carbon content), subbituminous coal (soft coal with intermediate carbon content), bituminous coal (soft coal with higher carbon and lower moisture content than subbituminous coal), and anthracite (hard coal with highest carbon content and lowest moisture content). Because anthracite is the most carbon-rich, moisture-deficient form of coal, it has the highest heating value.

Natural Gas Formation

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Most natural gas is formed from plankton—tiny water-dwelling organisms, including algae and protozoans—that accumulated on the ocean floor as they died. These organisms were slowly buried and compressed under layers of sediment. Over millions of years, the pressure and heat generated by overlying sediments converted this organic material into natural gas. Natural gas is composed primarily of methane and other light hydrocarbons. As discussed previously, natural gas frequently migrates through porous and fractured reservoir rock with petroleum and subsequently accumulates in underground reservoirs. Because of its light density relative to petroleum, natural gas forms a layer over the petroleum. Natural gas may also form in coal deposits, where it is often found dispersed throughout the pores and fractures of the coal bed.

Other Fossil Fuels

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Geologists have identified immense deposits of other hydrocarbons, including gas hydrates (methane and water), tar sands, and oil shale. Vast deposits of gas hydrates are contained in ocean sediments and in shallow polar soils. In these marine and polar environments, methane molecules are encased in a crystalline structure with water molecules. This crystalline solid is known as gas hydrate. Because technology for the commercial extraction of gas hydrates has not yet been developed, this type of fossil fuel is not included in most world energy resource estimates.

Tar sands are heavy, asphaltlike hydrocarbons found in sandstone. Tar sands form where petroleum migrates upward into deposits of sand or consolidated sandstone. When the petroleum is exposed to water and bacteria present in the sandstone, the hydrocarbons often degrade over time into heavier, asphaltlike bitumen. Oil shale is a fine-grained rock containing high concentrations of a waxy organic material known as kerogen. Oil shale forms on lake and ocean bottoms where dead algae, spores, and other microorganisms died millions of years ago and accumulated in mud and silt. The increasing pressure and temperature from the buildup of overlying sediments transformed the organic material into kerogen and compacted the mud and silt into oil shale. However, this pressure and heat was insufficient to chemically break down the kerogen into petroleum. Because the hydrocarbons contained in tar sand and oil shale are not fluids, these hydrocarbons are more difficult and costly to recover than liquid petroleum.

Removing and Refining Fossil Fuels

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Geologists use a variety of sophisticated instruments to locate underground petroleum, natural gas, and coal deposits. These instruments allow scientists to interpret the geologic composition, history, and structure of sedimentary basins in the earth’s crust. Once located, petroleum and natural gas deposits are removed by wells drilled down into the deposit, while coal is removed by excavation.

Petroleum and Natural Gas
To locate deposits of petroleum and natural gas, exploration geologists search for geologic regions containing the ingredients necessary for petroleum formation: organic-rich source rock, burial temperatures sufficiently high to generate petroleum from organic material, and petroleum-trapping rock formations.

When potentially petroleum-rich geologic formations are identified, wells are drilled into the sedimentary basin. If a well intersects porous reservoir rock containing significant petroleum and natural gas deposits, pressure inside the trap may force the liquid hydrocarbons spontaneously to the surface. However, pressure inside the trap typically declines to the point where the petroleum must be pumped to the surface.

Once petroleum has been extracted from the ground, it is transported by pipeline, truck, or tanker to a refinery to be separated into liquid and gas components. Raw petroleum is heated to distill hydrocarbons by molecular weight. Lighter molecules are separated and refined into gasoline and other fuels, while heavier molecules are processed into engine lubricants, asphalt, waxes, and other products. Because demand for fuel far exceeds demand for the products made from the heavier hydrocarbons, refiners often break apart the heavy molecules into lighter ones that can be refined into gasoline. They do so by means of processes called thermal cracking and catalytic cracking.

Coal
Because of their enormity, the world’s most extensive coal beds have already been identified. Modern underground mining commonly employs machines called longwall miners to remove coal. These machines use rotating drums studded with picks to rip coal from seams in large chunks.

Surface-mine operators use mammoth earth-moving shovels to mine coal. These shovels first remove overlying soil and rock so the coal beds can be blasted apart. The blasted coal is scooped up and loaded into the beds of huge trucks for transport.

Fossil Fuels: Commercial Uses

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Once fossil fuel has been extracted and processed, it can be burned for direct uses, such as to power cars or heat homes, or it can be combusted for the generation of electrical power.

Direct Combustion
Fossil fuels are primarily burned to produce energy. This energy is used to power automobiles, trucks, airplanes, trains, and ships around the world; to fuel industrial manufacturing processes; and to provide heat, light, air conditioning, and energy for homes and businesses.

To provide fuel for transportation, petroleum is refined into gasoline, diesel fuel, jet fuel, and other derivatives used in most of the world’s automobiles, trucks, trains, aircraft, and ships.

Demand for natural gas, historically considered a waste by-product of petroleum and coal mining, is growing in business and industry because it is a cleaner-burning fuel than petroleum or coal. Natural gas, which can be piped directly to commercial plants or individual residences and used on demand, is used for heating and for air conditioning. Residential uses of natural gas also include fuel for stoves and other heating appliances.

Electricity Generation
In addition to direct combustion for commercial uses, fossil fuels are also burned to generate most of the world’s electric power. In 2001 fossil fuel fired power plants produced 64 percent of the world’s electrical power, down from 71 percent in the late 1970s. In 2001 the world’s remaining electricity supply was generated primarily by hydroelectric power (17 percent) and nuclear fission (17 percent), with solar, geothermal, and other sources accounting for a relatively small amount.

Interstellar Matter

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Interstellar Matter, gas and dust between the stars in a galaxy. In our own galaxy, the Milky Way, we can see glowing gas and dark, obscuring dust between the galaxy’s many visible stars. This gas and dust makes up interstellar matter. Galaxies differ in the density of interstellar matter that they contain. Spiral galaxies, such as the Milky Way, have much more interstellar matter than elliptical galaxies, which have almost none. About 3 percent of the mass of the Milky Way Galaxy is interstellar gas, and 1 percent is interstellar dust. Stars make up the rest of the ordinary matter in the galaxy. Dark matter—a material that does not reflect or emit light or other forms of electromagnetic radiation—also makes up some of the mass of the galaxy. Astronomers consider interstellar matter separately from intergalactic matter, or matter between galaxies.

Hydrogen gas makes up most of the interstellar matter, but essentially all of the chemical elements occur in interstellar matter. About 90 percent of the atoms in space are hydrogen, about 9 percent helium, and less than 1 percent consists of all the other chemical elements. The interstellar matter is so spread out that the space it occupies would be considered a vacuum in laboratories on Earth.

Ultraviolet Radiation

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Ultraviolet Radiation, electromagnetic radiation that has wavelengths in the range between 4000 angstrom units (Å), the wavelength of violet light, and 150 Å, the length of X rays. Natural ultraviolet radiation is produced principally by the sun. Ultraviolet radiation is produced artificially by electric-arc lamps.

Ultraviolet radiation is often divided into three categories based on wavelength, UV-A, UV-B, and UV-C. In general shorter wavelengths of ultraviolet radiation are more dangerous to living organisms. UV-A has a wavelength from 4000 Å to about 3150 Å. UV-B occurs at wavelengths from about 3150 Å to about 2800 Å and causes sunburn; prolonged exposure to UV-B over many years can cause skin cancer. UV-C has wavelengths of about 2800 Å to 150 Å and is used to sterilize surfaces because it kills bacteria and viruses.

The earth's atmosphere protects living organisms from the sun's ultraviolet radiation. If all the ultraviolet radiation produced by the sun were allowed to reach the surface of the earth, most life on earth would probably be destroyed. Fortunately, the ozone layer of the atmosphere absorbs almost all of the short-wavelength ultraviolet radiation, and much of the long-wavelength ultraviolet radiation. However, ultraviolet radiation is not entirely harmful; a large portion of the vitamin D that humans and animals need for good health is produced when the human's or animal's skin is irradiated by ultraviolet rays.

When exposed to ultraviolet light, many substances behave differently than when exposed to visible light. For example, when exposed to ultraviolet radiation, certain minerals, dyes, vitamins, natural oils, and other products become fluorescent—that is, they appear to glow. Molecules in the substances absorb the invisible ultraviolet light, become energetic, then shed their excess energy by emitting visible light. As another example, ordinary window glass, transparent to visible light, is opaque to a large portion of ultraviolet rays, particularly ultraviolet rays with short wavelengths. Special-formula glass is transparent to the longer ultraviolet wavelengths, and quartz is transparent to the entire naturally occurring range.

In astronomy, ultraviolet-radiation detectors have been used since the early 1960s on artificial satellites, providing data on stellar objects that cannot be obtained from the earth's surface. An example of such a satellite is the International Ultraviolet Explorer, launched in 1978.

Air

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Air, mixture of gases that composes the atmosphere surrounding Earth. These gases consist primarily of the elements nitrogen, oxygen, argon, and smaller amounts of hydrogen, carbon dioxide, water vapor, helium, neon, krypton, xenon, and others. The most important attribute of air is its life-sustaining property. Human and animal life would not be possible without oxygen in the atmosphere. In addition to providing life-sustaining properties, the various atmospheric gases can be isolated from air and used in industrial and scientific applications, ranging from steelmaking to the manufacture of semiconductors. This article discusses how atmospheric gases are isolated and used for industrial and scientific purposes.

GASES IN THE ATMOSPHERE

The atmosphere begins at sea level, and its first layer, the troposphere, extends from 8 to 16 km (5 and 10 mi) from Earth’s surface. The air in the troposphere consists of the following proportions of gases: 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, 0.03 percent carbon dioxide, and the remaining 0.07 percent is a mixture of hydrogen, water, ozone, neon, helium, krypton, xenon, and other trace components. Companies that isolate gases from air use air from the troposphere, so they produce gases in these same proportions.

PURIFYING AIR

Most larger air-separation plants continue to use cryogenic distillation to separate air gases. Before pure gases can be isolated from air, unwanted components such as water vapor, dust, and carbon dioxide must be removed. First, the air is filtered to remove dust and other particles. Next, the air is compressed as the first step in liquefying the air. However, as the air is compressed, the molecules begin striking each other more frequently, raising the air’s temperature. To offset the higher temperatures, water heat exchangers cool the air both during and after compression. As the air cools, most of its water vapor content condenses into liquid and is removed.

After being compressed, the air passes through beds of adsorption beads that remove carbon dioxide, the remaining water vapor, and molecules of heavy hydrocarbons, such as acetylene, butane, and propylene. These compounds all freeze at a higher temperature than do the other air gases. They must be removed before the air is liquefied or they will freeze in the column where distillation occurs.

COMPRESSED AIR

Not all industrial uses of air require it to be separated into its component gases. Compressed air—plain air that has been pressurized by squeezing it into a smaller-than-normal volume—is used in many industrial applications. When air is compressed, the gas molecules collide with each other more frequently and with more force, producing higher kinetic energy. The kinetic energy in compressed air can be converted into mechanical energy or it can be used to produce a powerful air flow or an air cushion. Compressed air is easily transmitted through pipes and hoses with little loss of energy, so it can be utilized at a considerable distance from the compressor or pressure tank.

Alabaster

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Alabaster, varietal name applied to two different minerals. One, Oriental alabaster, was extensively used by the ancient Egyptians. It is a variety of calcite, with a hardness of 3; it is usually white and translucent, but is often banded with dark or colored streaks. The other mineral, true alabaster, is a variety of gypsum, usually snow-white in color with a uniform, fine grain. True alabaster is softer than Oriental alabaster; it has a hardness of 1.5 and is easily carved into intricate shapes. Deposits of fine gypsum alabaster are found in Italy, England, Iran, and Pakistan.

Alkalies

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Alkalies or Alkalis (Arabic al-qili, “ashes of the saltwort plant”), originally the hydroxides and carbonates of potassium and sodium, leached from plant ashes. The term now applies to the corresponding compounds of ammonium, NH4, and the other alkali metals and to the hydroxides of calcium, strontium, and barium. All of these substances produce hydroxide ions, OH-, when dissolved in water. The carbonates and ammonium hydroxide give only moderate concentrations of hydroxide ions and are termed mild alkalis. The hydroxides of sodium and potassium, however, produce hydroxide ions in high enough concentration to destroy flesh; for this reason they are called caustic alkalis. Solutions of alkalis turn red litmus blue, react with and neutralize acids, feel slippery, and are electrical conductors.

Caustic soda, or sodium hydroxide, NaOH, is an important commercial product, used in making soap, rayon, and cellophane; in processing paper pulp; in petroleum refining; and in the manufacture of many other chemical products. Caustic soda is manufactured principally by electrolysis of a common salt solution, with chlorine and hydrogen as important by-products.

Sodium carbonate, Na2CO3, one of the mild alkalis, is manufactured from natural deposits or made from common salt brines by the Solvay process. It is used in the manufacture of glass and as a cleaning agent and water softener.

Alum

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Alum, any of a group of chemical compounds, made up of water molecules and two kinds of salts, one of which is usually aluminum sulfate combined in definite proportions. Potassium alum, also known as common alum, is the most important type of alum.

Potassium alum is a colorless substance that forms large octahedral or cubic crystals when potassium sulfate and aluminum sulfate are dissolved together and the solution is cooled. The solutions of potassium alum are acidic.

Potassium alum is soluble in seven times its weight of water at room temperature and is very soluble in hot water. When crystalline potassium alum is heated, some of the water of hydration becomes chemically separated, and the partly dehydrated salt dissolves in this water, so that the alum appears to melt at about 90°C (about 194°F). When heated to about 200°C (about 392°F), potassium alum swells up, loses all the water and some sulfur trioxide, and becomes a basic salt called burnt alum. Potassium alum has a density of 1.725.

Other types of alums made with aluminum sulfate include sodium alum, ammonium alum, and silver alum. Alums are used for flameproofing textiles and in baking powders, mordants for delicate dyeing operations, and medicines. Potassium alum is a powerful

Alumina

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Alumina or Aluminum Oxide, an oxide found in nature as the minerals corundum, diaspore, gibbsite, and most commonly, bauxite, an impure form of gibbsite. It is the only oxide formed by the metal aluminum. The precious stones ruby and sapphire are composed of corundum colored by small amounts of impurities.

Fused alumina, alumina that has been melted and recrystallized, is identical in chemical and physical properties with natural corundum. It is exceeded in hardness only by diamond and by a few synthetic substances, notably carborundum, or silicon carbide. Both impure natural corundum (emery) and pure synthetic corundum (Alundum) are used as abrasives. At room temperature alumina is insoluble in all ordinary chemical reagents. Its melting point is high, slightly above 2000°C (3632°F), and so alumina is useful as a refractory, for example, for the linings of special furnaces.

Alumina can be purified by fusing it with sodium carbonate. The resulting sodium aluminate is dissolved in water, leaving impurities, such as iron, as an insoluble residue. Hydrated alumina is reprecipitated from the solution by carbon dioxide. Because the alumina contained in bauxite is soluble in sodium hydroxide solution, a less expensive method may be used. By alternately concentrating and diluting the solution, hydrated alumina is precipitated, and the sodium hydroxide may be reused without neutralization. Hydrated alumina, also called aluminum hydroxide or aluminum hydrate, is readily soluble in acids or alkalies and is used as a raw material in the manufacturing process of all aluminum compounds.

Animal Fibers

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All animal fibers are complex proteins. They are resistant to most organic acids (see Acids and Bases) and to certain powerful mineral acids such as sulfuric acid. However, protein fibers are damaged by mild alkalies (basic substances) and may be dissolved by strong alkalies such as sodium hydroxide. They can also be damaged by chlorine-based bleaches, and undiluted liquid hypochloride bleach will dissolve wool or silk.

The principal component of silk is the protein fibroin. Silk is exuded in continuous filaments from the abdomens of various insects and spiders. It is the only natural filament that commonly reaches a length of more than 1000 m (more than 3300 ft). The only silk used in commercial textiles is produced from the cocoons of the silkworm. Several silk filaments can be gathered to produce textile yarn. However, silk is often produced and used in staple form to manufacture spun yarns.

The principal component of hair, wool, and fur is the protein keratin. Individual hairs may be as long as 91 cm (36 in) but are usually no more than 41 cm (16 in). Thus, fibers of hair and wool are not continuous and must be spun into yarn if they are to be woven or knitted into textile fabrics, or they must be made into felt. Any hair fiber can legally be marketed as wool or bear the common English name of the animal from which it was gathered—for example, camel's hair.

The principal hair fiber used to produce textile fabrics is sheep's wool. In wild sheep, the wool is a short, soft underlayer protected by longer, coarser hair. In domesticated sheep bred for their fleece, the wool is much longer. Yarns made of wool are classified as either woolen or worsted. Wool fibers less than 5 cm (less than 2 in) in length are made into fuzzy, soft woolen yarns. Longer fibers are used for the smoother and firmer worsted yarns. Naturally crimped wool fibers produce air-trapping yarns that are used for insulating materials.

Other animals used as sources of hair fibers for textiles include camels, llamas, alpacas, guanacos, vicuñas, rabbits, reindeer, Angora goats, and Kashmīr (or cashmere) goats. Fur fibers from animals such as mink and beavers are sometimes blended with other hairs to spin luxury yarns but are most often found as fur pelts. Horsehair and cow's hair are used for felts and are sometimes spun as yarn, particularly for upholstery and other applications for which durability is important. Even human hair has been spun into yarn and used for textiles.

Keratin

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Keratin, highly fibrous and resistant protein that makes up most of the material in the cells forming the epidermis, hair, nails, scales, feathers, beaks, horns, and hooves of animals. These cells originate from permanent populations of germinal cells, and as they migrate outward they undergo specific patterns of differentiation in a process called keratinization. That is, the cells become increasingly filled with microfibrils of keratin, and the nuclei and organelles of the cells are reabsorbed. Little is understood, however, about how the cells differentiate to form such diverse and efficient structures as the elastic outer layer of the skin of mammals or the stiff scales of fish. An important quality of keratin is its ability to extend and contract reversibly.

Vegetable Fibers

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Vegetable fibers are predominantly cellulose, which, unlike the protein of animal fibers, resists alkalies. Vegetable fibers resist most organic acids but are destroyed by strong mineral acids. Improper use of most bleaches can also weaken or destroy these fibers.

There are four major types of vegetable fibers: seed fibers, which are the soft hairs that surround the seeds of certain plants; bast fibers, the tough fibers that grow between the bark and stem of many dicotyledonous plants (see Dicots); vascular fibers, the tough fibers found in the leaves and stems of monocotyledons (see Monocots); and grass-stem fibers. Other fiber types, of limited utility, include strips of leaf skins, such as raffia; the fiber of fruit cases, such as coir; and palm fibers.

Only two seed fibers, cotton and kapok (see Ceiba), have commercial importance. Cotton fiber, which grows in the seed pod of cotton plants, is the only one that is useful for the manufacture of textiles. Different species of cotton plants produce fibers of different lengths. Long-staple fibers are spun into fine, strong yarns, which are then woven into better-quality fabrics. Short-staple fibers produce coarser yarns for durable fabrics. Cotton yarns can be dyed (see Dyeing) and printed easily, so that they are useful for producing woven fabrics with a multitude of colors and designs. Kapok cannot be spun but is used as upholstery stuffing. Because it is hollow, kapok is buoyant. It was once used in flotation devices such as life preservers, but it has largely been replaced by other materials.

A wide variety of bast fibers are used in applications ranging from fine woven textiles to cordage. Linen cloth is made from flax. Coarser clothes and rope are produced from hemp, jute, ramie, and sunn.

Vascular fibers are used almost exclusively for making cordage. They include agave (sisal), henequen, manila hemp, and yucca. The vascular fibers of pineapple have been used in the production of textiles. Entire stems of some grasses and straws, such as esparto, are woven as fibers for hats and matting.

Antifreeze

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Antifreeze, chemical substance added to a liquid to lower its freezing point. It prevents the freezing of the coolants used in airplane, automobile, and tractor engines, in refrigeration liquids, and in snow-melting and deicing agents.

The ideal antifreeze should be chemically stable, be miscible in the coolant, have low viscosity and electrical conductivity and a high boiling point, be noncorrosive, and have good heat-transfer properties. The most widely used antifreeze materials in automotive engines today are methyl alcohol, ethyl alcohol, and ethylene glycol; most of them contain a phosphate, nitrate, or other anticorrosive agent.

Antimatter

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Antimatter, matter composed of elementary particles that are, in a special sense, mirror images of the particles that make up ordinary matter as it is known on earth. Antiparticles have the same mass as their corresponding particles but have opposite electric charges or other properties related to electromagnetism. For example, the antimatter electron, or positron, has opposite electric charge and magnetic moment (a property that determines how it behaves in a magnetic field), but is identical in all other respects to the electron. The antimatter equivalent of the chargeless neutron, on the other hand, differs in having a magnetic moment of opposite sign (magnetic moment is another electromagnetic property). In all of the other parameters involved in the dynamical properties of elementary particles, such as mass, spin, and partial decay, antiparticles are identical with their corresponding particles.

The existence of antiparticles was first proposed by the British physicist Paul Adrien Maurice Dirac, arising from his attempt to apply the techniques of relativistic mechanics (see Relativity) to quantum theory. In 1928 he developed the concept of a positively charged electron but its actual existence was established experimentally in 1932. The existence of other antiparticles was presumed but not confirmed until 1955, when antiprotons and antineutrons were observed in particle accelerators. Since then, the full range of antiparticles has been observed or indicated. Antimatter atoms were created for the first time in September 1995 at the European Organization for Nuclear Research (CERN). Positrons were combined with antimatter protons to produce antimatter hydrogen atoms. These atoms of antimatter exist only for forty-billionths of a second, but physicists hope future experiments will determine what differences there are between normal hydrogen and its antimatter counterpart.

A profound problem for particle physics and for cosmology in general is the apparent scarcity of antiparticles in the universe. Their nonexistence, except momentarily, on earth is understandable, because particles and antiparticles are mutually annihilated with a great release of energy when they meet. Distant galaxies could possibly be made of antimatter, but no direct method of confirmation exists. Most of what is known about the far universe arrives in the form of photons, which are identical with their antiparticles and thus reveal little about the nature of their sources. The prevailing opinion, however, is that the universe consists overwhelmingly of “ordinary” matter, and explanations for this have been proposed by recent cosmological theory.

Apatite

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Apatite (Greek apate, “deception”), mineral so named because it resembles various other minerals for which it might be mistaken. It consists chiefly of phosphate of lime. Apatite is a distinct mineral of composition in which some or all of the fluorine may be replaced by chlorine (chlorapatite). The mineral crystallizes in the hexagonal system (see Crystal) and has a hardness of 5 and a specific gravity of 3.2. When pure, apatite is colorless and transparent, but it may exhibit various degrees of color and opacity. These mineral phosphates of lime were often used in the preparation of fertilizers, but they have been replaced by phosphate rock.

Aqua Regia

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Aqua Regia (Latin, “royal water”), mixture of concentrated hydrochloric and nitric acids, containing one part by volume of nitric acid to three parts of hydrochloric acid. Aqua regia was used by the alchemists and its name is derived from its ability to dissolve the so-called noble metals, particularly gold, which are inert to either of the acids used separately. It is still occasionally used in the chemical laboratory for dissolving gold and platinum.

Avogadro’s Number

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Avogadro’s Number, the number of molecules that exist in one mole, or gram molecular weight, of any substance. One gram molecular weight is the weight of a substance, in grams, that is numerically equivalent to the dimensionless molecular weight of that substance (see Periodic Law). The number of molecules in one gram molecular weight has been determined to be approximately molecules, as established by various methods currently available to physical chemists.

The Avogadro number is named in honor of the Italian physicist Amedeo Avogadro, who postulated in 1811 that equal volumes of gases, at equivalent temperatures and pressures, contain the same number of molecules (see Avogadro's Law). The theory was significant in the development of chemistry, but the number itself was not calculated until the later 19th century, when the concept was extended to include not only gases but all chemicals. Volume considerations do not apply to liquids or solids, but Avogadro's number itself holds true for all substances, whatever their state.

Avogadro’s Law

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Avogadro’s Law, fundamental law of chemistry stating that under identical conditions of temperature and pressure, equal volumes of gases contain an equal number of molecules. The law was first proposed as a hypothesis by the Italian physicist Amedeo Avogadro in 1811. Italian chemists and physicists continued to develop this hypothesis, and in the 1850s, largely through the efforts of the Italian chemist Stanislao Cannizzaro, Avogadro's law was universally accepted.

Catalysis

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Catalysis, alteration of the speed of a chemical reaction, through the presence of an additional substance, known as a catalyst, that remains chemically unchanged by the reaction. Enzymes, which are among the most powerful catalysts, play an essential role in living organisms, where they accelerate reactions that otherwise would require temperatures that would destroy most of the organic matter.

A catalyst in a solution with—or in the same phase as—the reactants is called a homogeneous catalyst. The catalyst combines with one of the reactants to form an intermediate compound that reacts more readily with the other reactant. The catalyst, however, does not influence the equilibrium of the reaction, because the decomposition of the products into the reactants is speeded up to a similar degree. An example of homogeneous catalysis is the formation of sulfur trioxide by the reaction of sulfur dioxide with oxygen, in which nitrogen dioxide serves as a catalyst. Under extreme heat, sulfur dioxide and nitrogen dioxide react to form sulfur trioxide and the intermediate compound nitric oxide, which then reacts with oxygen to re-form nitrogen dioxide. The same amount of nitrogen dioxide exists at both the beginning and end of the reaction.

Battery

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Battery, also electric cell, device that converts chemical energy into electricity. Strictly speaking, a battery consists of two or more cells connected in series or parallel, but the term is also used for single cells. All cells consist of a liquid, paste, or solid electrolyte and a positive electrode, and a negative electrode. The electrolyte is an ionic conductor; one of the electrodes will react, producing electrons, while the other will accept electrons. When the electrodes are connected to a device to be powered, called a load, an electrical current flows.

Batteries in which the chemicals cannot be reconstituted into their original form once the energy has been converted (that is, batteries that have been discharged) are called primary cells or voltaic cells. Batteries in which the chemicals can be reconstituted by passing an electric current through them in the direction opposite that of normal cell operation are called secondary cells, rechargeable cells, storage cells, or accumulators.

SOLAR BATTERY

Solar batteries produce electricity by a photoelectric conversion process. The source of electricity is a photosensitive semiconducting substance such as a silicon crystal to which impurities have been added. When the crystal is struck by light, electrons are dislodged from the surface of the crystal and migrate toward the opposite surface. There they are collected as a current of electricity. Solar batteries have very long lifetimes and are used chiefly in spacecraft as a source of electricity to operate the equipment aboard. See Solar Energy.

Beryl

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Beryl, mineral and, in certain varieties, a valuable gem material. Chemically it consists of aluminum beryllium silicate and it is the chief commercial ore of beryllium. Pure beryl is colorless and transparent. Emerald, one of the most valuable gems, is a variety that is colored green by minute amounts of chromium. Aquamarine, also a gemstone, is a blue beryl, more common than emerald. Golden beryl and morganite or rose beryl are less valuable. Colorless beryl is occasionally used as a gem under the name goshenite. Beryl has a vitreous luster with little fire or brilliancy, and so its value depends principally on hardness, transparency, and color. It has a hardness of 7.5 to 8 and a specific gravity of 2.75 to 2.8.

Beryl crystallizes in the hexagonal system. Large lettuce-green opaque crystals, some weighing over a ton, are found embedded in a variety of granite called pegmatite. Large, transparent crystals of the colored varieties are occasionally found.

Biological Radiation Effects

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Biological Radiation Effects, effects observed when ionizing radiation strikes living tissue and damages the molecules of cellular matter. Cellular function may be temporarily or permanently impaired from the radiation, or the cell may be destroyed. The severity of the injury depends on the type of radiation, the absorbed dose, the rate at which the dose was absorbed, and the radiosensitivity of the tissues involved. The effects are the same, whether from a radiation source outside the body or from material within.

The biological effects of a large dose of radiation delivered rapidly differ greatly from those of the same dose delivered slowly. The effects of rapid delivery are due to cell death, and they become apparent within hours, days, or weeks. Protracted exposure is better tolerated because some of the damage is repaired while the exposure continues, even if the total dose is relatively high. If the dose is sufficient to cause acute clinical effects, however, repair is less likely and may be slow even if it does occur. Exposure to doses of radiation too low to destroy cells can induce cellular changes that may be detectable clinically only after some years.

Biomass

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Biomass, contraction for biological mass, the amount of living material provided by a given area of the earth's surface. The term is most familiar from discussions of biomass energy, that is, the fuel energy that can be derived directly or indirectly from biological sources. Biomass energy from wood, crop residues, and dung remains the primary source of energy in developing regions. In a few instances it is also a major source of power, as in Brazil, where sugarcane is converted to ethanol fuel, and in China's Sichuan province, where fuel gas is obtained from dung. Various research projects aim at further development of biomass energy, but economic competition with petroleum has mainly kept such efforts at an early developmental stage.

See Synthetic Fuels; Gasohol.

Boric Acid

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Boric Acid, white crystalline powder. Although boric acid is poorly soluble in water at room temperature (1 g dissolving in 18 g water), it dissolves readily in hot water (1 g dissolving in less than 4 g water) and in alcohol and glycerine. It is slightly volatile in steam. A significant amount of boric acid appears in natural steam vents in Tuscany (Toscana), Italy, but free boric acid is not otherwise found in nature. Salts, however, occur in many places. Boric acid can be easily prepared by treating borax with sulfuric acid. Boric acid in solution is only slightly acidic and acts as a nonirritating, slightly astringent antiseptic, mild enough to be used as an eyewash. Commercially, boric acid is used in glazing pottery, in fireproofing cloth, in making electroplating baths and artificial gems, and in hardening steels.

Boson

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Boson, one of the two basic divisions of elementary particles, the basic units of matter and energy. Some bosons, called elementary bosons, are fundamental particles, meaning they cannot be divided into anything smaller. These bosons carry energy between particles of matter, affecting the behavior of matter particles and holding the particles together in larger structures. Mesons are bosons that are made of more than one particle. Bosons are named for Indian physicist Satyendra Bose, who (with German-born American physicist Albert Einstein) developed a set of equations that describe the way bosons behave. See also Elementary Particles.

Bosons fall into two main groups. One group contains the elementary bosons, or bosons that are not made up of other particles. Elementary bosons play a crucial role in transferring energy between the fermions that compose matter. The other group is called the mesons. Mesons are composite particles—that is, they are made up of other particles. Mesons play an important role in holding together the particles in atoms.

Brass

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Brass (alloy), alloy of copper and zinc. Harder than copper, it is ductile and can be hammered into thin leaves. Formerly any alloy of copper, especially one with tin, was called brass, and it is probable that the “brass” of ancient times was of copper and tin. The modern alloy came into use about the 16th century.

The malleability of brass varies with its composition and temperature and with the presence of foreign metals, even in minute quantities. Some kinds of brass are malleable only when cold, others only when hot, and some are not malleable at any temperature. All brass becomes brittle if heated to a temperature near the melting point.

To prepare brass, zinc is mixed directly with copper in crucibles or in a reverberatory or cupola furnace. The ingots are rolled when cold. The bars or sheets can be rolled into rods or cut into strips that can be drawn out into wire.

British Thermal Unit

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British Thermal Unit, in science and engineering, a unit measurement of heat or energy, usually abbreviated as Btu or BTU. One Btu was originally defined as the quantity of heat required to raise the temperature of 1 lb (0.45 kg) of water from 59.5° F (15.3° C) to 60.5° F (15.8° C) at constant pressure of 1 atmosphere; for very accurate scientific or engineering measurements, however, this value was not precise enough. The Btu has now been redefined in terms of the joule as equal to 1055 joules; in engineering, a Btu is equivalent to approximately 0.293 watt-hour.

Bronze

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Bronze, metal compound containing copper and other elements. The term bronze was originally applied to an alloy of copper containing tin, but the term is now used to describe a variety of copper-rich material, including aluminum bronze, manganese bronze, and silicon bronze.

CHARACTERISTICS AND USES

Bronze is stronger and harder than any other common metal alloy except steel. It does not easily break under stress, is corrosion resistant, and is easy to form into finished shapes by molding, casting, or machining.

The strongest bronze alloys contain tin and a small amount of lead. Tin, silicon, or aluminum is often added to bronze to improve its corrosion resistance. As bronze weathers, a brown or green film forms on the surface. This film inhibits corrosion. For example, many bronze statues erected hundreds of years ago show little sign of corrosion. Bronzes have a low melting point, a characteristic that makes them useful for brazing—that is, for joining two pieces of metal. When used as brazing material, bronze is heated above 430°C (800°F), but not above the melting point of the metals being joined. The molten bronze fuses to the other metals, forming a solid joint after cooling.

Lead is often added to make bronze easier to machine. Silicon bronze is machined into piston rings and screening, and because of its resistance to chemical corrosion it is also used to make chemical containers. Manganese bronze is used for valve stems and welding rods. Aluminum bronzes are used in engine parts and in marine hardware.

Bronze containing 10 percent or more tin is most often rolled or drawn into wires, sheets, and pipes. Tin bronze, in a powdered form, is sintered (heated without being melted), pressed into a solid mass, saturated with oil, and used to make self-lubricating bearings.

Positron

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Positron, elementary particle identical to the electron except for its electric charge and its magnetic moment (a property that determines how it behaves in a magnetic field). Positrons are elementary particles, which are fundamental constituents of matter—that is, they cannot be divided into smaller units. Positrons have uses in medicine and in industry, particularly in a form of imaging known as positron emission tomography (PET).

CHARACTERISTICS AND BEHAVIOR

All elementary particles have basic characteristics called mass, charge, and spin (a property analogous to angular momentum). The positron has the same mass—amount of matter—as the electron, and the same spin. The two particles also have the same amount of electric charge, but the positron’s charge is positive and the electron’s is negative. For this reason, the positron is sometimes called a positive electron. Although positrons and electrons have a measurable mass, charge, and spin, they have no measurable size, shape, or structure. Scientists therefore consider them pointlike. Other pointlike elementary particles include neutrinos and quarks.

Every elementary particle has an equal and opposite antiparticle. The positron is the antiparticle of the electron. Just as particles combine to form ordinary matter, antiparticles combine to create antimatter. When a particle and its antiparticle collide, they destroy each other, releasing energy. This feature makes positrons useful in creating PET scans, images of the brain and other soft tissues inside the body. To create a PET scan, positron-emitting substances are injected into the body. Computers track the energy released inside the body by positron-electron collisions and use this information to form images. PET scans are especially helpful in identifying and locating brain tumors and in studying other disorders in the brain. Positrons are also used in industry to reveal defects on metal surfaces and in semiconductors.

Positrons are emitted by certain radioactive substances that scientists create in the laboratory. They are also produced within stars and by collisions of cosmic rays (high energy particles that originate in space). But positrons are short-lived because they soon collide with electrons. In the laboratory, scientists create positrons by a method known as pair production. In this method a gamma ray (particle of electromagnetic energy) interacts with the nucleus of a very heavy atom, producing a positron and an electron.

Calcite

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Calcite, an extremely abundant mineral composed of calcium carbonate. It can form crystals in a wide variety of shapes and colors. It can be a primary or secondary component in sedimentary, igneous, or metamorphic rocks. It often provides the cement that binds particles together in sedimentary rocks. Calcite exhibits several physical properties that make it relatively easy to identify. These properties include its tendency to react with a dilute solution of hydrochloric acid and to break into rhombohedrons. Rhombohedrons are six-sided solids that resemble cubes except that the faces meet at 60° instead of 90°. Calcite crystals and calcite-rich rocks are valuable for a variety of uses that range from components in optical instruments to cement.

Calcite is the third most common mineral in the earth’s crust (behind feldspar and quartz). Because of its abundance, calcite can be found in many rock types.

As a crystal, calcite can take on a variety of forms, also called “habits”. More than 300 different forms of calcite exist. Some especially common shapes are “dogtooth spar” and rhombohedrons. Dogtooth spar crystals are elongated six-sided pyramids, except without a flat bottom. Rhombohedrons resemble cubes except that their faces intersect at 60° instead of 90°. Most large crystals of calcite form in veins by precipitation from hot or cold groundwater as it moves through cracks in rock deep underground. Most large masses of calcite-rich rock form biochemically, in the case of limestone, or as a precipitate, in the case of travertine.

Capacitance

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Capacitance, ability of a circuit system to store electricity. The capacitance of a capacitor is measured in farads and is determined by the formula C = q/V, where q is the charge (in coulombs) on one of the conductors and V is the potential difference (in volts) between the conductors. The capacitance depends only on the thickness, area, and composition of the capacitor's dielectric.

Capacitor

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Capacitor, or electrical condenser, device for storing an electrical charge. In its simplest form a capacitor consists of two metal plates separated by a nonconducting layer called the dielectric. When one plate is charged with electricity from a direct-current or electrostatic source, the other plate will have induced in it a charge of the opposite sign; that is, positive if the original charge is negative and negative if the charge is positive. The Leyden jar is a simple form of capacitor in which the two conducting plates are metal-foil coatings on the inside and outside of a glass bottle or jar that serves as the dielectric. The electrical size of a capacitor is its capacitance, the amount of electric charge it can hold.

Capacitors are limited in the amount of electric charge they can absorb; they can conduct direct current for only an instant but function well as conductors in alternating-current circuits. This property makes them useful when direct current must be prevented from entering some part of an electric circuit. Fixed-capacity and variable-capacity capacitors are used in conjunction with coils as resonant circuits in radios and other electronic equipment. Large capacitors are also employed in power lines to resonate the load on the line and make it possible for the line to transmit more power.

Capacitors are produced in a wide variety of forms. Air, mica, ceramics, paper, oil, and vacuums are used as dielectrics, depending on the purpose for which the device is intended.

Carbon Dioxide

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Carbon Dioxide, colorless, odorless, and slightly acid-tasting gas, sometimes called carbonic acid gas, the molecule of which consists of one atom of carbon joined to two atoms of oxygen. It was called “fixed air” by the Scottish chemist Joseph Black, who obtained it through the decomposition of chalk and limestone and recognized that it entered into the chemical composition of these substances. The French chemist Antoine Lavoisier proved that it is an oxide of carbon by showing that the gas obtained by the combustion of charcoal is identical in its properties with the “fixed air” obtained by Black. Carbon dioxide is about 1.5 times as dense as air. It is soluble in water, 0.9 volume of the gas dissolving in 1 volume of water at 20° C (68° F).

Carbon dioxide is produced in a variety of ways: by combustion, or oxidation, of materials containing carbon, such as coal, wood, oil, or foods; by fermentation of sugars; and by decomposition of carbonates under the influence of heat or acids. Commercially, carbon dioxide is recovered from furnace or kiln gases; from fermentation processes; from reaction of carbonates with acids; and from reaction of steam with natural gas, a step in the commercial production of ammonia. The carbon dioxide is purified by dissolving it in a concentrated solution of alkali carbonate or ethanolamine and then heating the solution with steam. The gas is evolved and is compressed into steel cylinders.

The atmosphere contains carbon dioxide in variable amounts, usually 3 to 4 parts per 10,000, and has been increasing by 0.4 percent a year. It is used by green plants in the process known as photosynthesis, by which carbohydrates are manufactured.

The presence of carbon dioxide in the blood stimulates breathing. For this reason, carbon dioxide is added to oxygen or ordinary air in artificial respiration and to the gases used in anesthesia.

Carbon Disulfide

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Carbon Disulfide, colorless, extremely volatile and flammable compound with a disagreeable, fetid odor. It is used as a solvent for oils, fats, and waxes; as a reagent in the manufacture of regenerated cellulose; as the starting material in the manufacture of carbon tetrachloride; in rayon and cellophane production; and in the vulcanization of rubber. Carbon disulfide is made by heating carbon and sulfur together or by the reaction between methane and sulfur vapor. It freezes at -111.53° C (-168.75° F) and boils at 46.25° C (115.25° F).

Chloroform

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Chloroform, name given to trichloromethane because of its supposed relation to formic acid. A colorless liquid, half again as dense as water and of about the same viscosity, chloroform has a heavy, etherlike odor and a burning sweetness of taste, being about 40 times as sweet as cane sugar. It is almost insoluble in water, but it is freely miscible with organic solvents and is an important solvent for gums, resins, fats, elements such as sulfur and iodine, and a wide variety of organic compounds.

Chloroform may be prepared by the chlorination of ethyl alcohol or of methane, or by the action of iron and acid on carbon tetrachloride; the latter is the principal industrial method in current use.

Chloroform was first prepared in 1831 and was first used as an anesthetic in 1847 in one of the earliest experiments on surgical anesthesia. In the presence of light, however, it tends to decompose, yielding the highly poisonous compound phosgene. Even when pure, it causes fatal cardiac paralysis in about one out of 3000 cases, and so is seldom used for anesthesia.

Chromite

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Chromite, only ore mineral of chromium, consisting of ferrous chromite and belonging to the spinel group. It crystallizes in the isometric system (see Crystal) and has a hardness of 5.5 and a specific gravity, or relative density, of 4.1 to 4.8. It is found in irregular brownish-black or black grains or octahedral crystals. Chromite is one of the first minerals to crystallize from magma. It occurs principally in rocks containing various amounts of ferromagnetic minerals. It also occurs in the minerals serpentine and peridotite, as well as in glacial and alluvian deposits. Large deposits of chromite are found in Kazakhstan, Turkey, and Zimbabwe, and also in Austria, Bosnia and Herzegovina, Serbia and Montenegro (formerly the Federal Republic of Yugoslavia), and the Former Yugoslav Republic of Macedonia.

Cinnabar

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Cinnabar, mineral consisting of mercuric sulfide, the principal commercial source of mercury. It is bright red in color, crystallizes in the hexagonal system (see Crystal), and has perfect prismatic cleavage. The hardness of cinnabar is 2.5, and the specific gravity is 8.10. The mineral is comparatively rare and usually occurs in volcanic vein deposits in sedimentary rocks. Important deposits of cinnabar are found in Spain, Italy, Mexico, and in California and Nevada in the United States. Artificial cinnabar, made from a mixture of sulfur and mercury, is used as the red pigment called vermilion.

Citric Acid

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Citric Acid, white solid, soluble in water and slightly soluble in organic solvents, which melts at 153° C (307° F). Aqueous solutions of citric acid are slightly more acidic than solutions of acetic acid. Traces of citric acid are found in numerous plants and animals, because it is a nearly universal intermediate product of metabolism. Large amounts of the acid are found in the juice of citrus fruits, from which it is precipitated by the addition of lime; the resulting calcium citrate is treated with sulfuric acid to regenerate the citric acid. Fermentation of sugar by the mold Aspergillus niger is the chief commercial source of the acid. It is added to some foods and beverages to produce a pleasant acid flavor; it is also used in medicines, in making blueprint paper, in textile printing, and as a polishing agent for metals.

Ether

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Ether (physics and astronomy), substance once thought to fill all space, but now known not to exist. Scientists of the late 19th and early 20th centuries believed that the ether was the medium, or substance, that allowed light to travel through space. The theory of relativity of German American physicist Albert Einstein showed that light did not need a medium through which to travel, so belief in the existence of the ether was abandoned. See also Quantum Theory.

DEVELOPMENT OF THE ETHER THEORY

Physicists have tried for hundreds of years to determine whether light is a stream of particles or a set of waves. In the 1860s and 1870s Scottish physicist James Clerk Maxwell formulated a theory that linked electricity and magnetism, and light, to waves of electromagnetic energy. His theory predicted that waves of varying electric and magnetic fields travel through space in the form of electromagnetic waves. These waves carry energy from place to place. Maxwell showed that these electromagnetic waves traveled at 300,000 km/s (190,000 mi/s)—the same speed that earlier scientists had measured to be the speed of light. His theory was thus strong evidence that light is carried by waves.

Before Maxwell’s theory was established, all of the types of waves that scientists knew of needed a medium—a substance through which to travel. For example, waves on a rope travel along the rope, and sound waves travel through air or some other substance. Scientists deduced that light waves must also travel through a medium. Scientists knew that light waves reached the earth from distant stars, and so they knew that light could travel through outer space. Physicists reasoned that outer space must be filled with an invisible medium, which they called the luminiferous ether, or just the ether. The function of the ether was to allow light waves to travel through space.

REFUTING THE ETHER THEORY

Scientists believed that the ether did not have mass and was invisible, so it was undetectable by normal chemical and physical means, even though it permeated all matter and all space. Albert A. Michelson, one of the first great American experimental physicists, formulated an experiment in the 1870s to detect the ether by studying its effect on light. Michelson repeated this experiment more accurately in 1887 with American chemist Edward W. Morley, and the experiment is known today as the Michelson-Morley experiment.

Michelson deduced that the earth must move relative to the ether. The earth moves through space around the sun, so if the ether exists and occupies all space, the earth must move through the ether. Michelson and Morley reasoned that the velocity of a light wave should depend on whether it is moving in the same direction as the ether (opposite the earth’s motion) or in the opposite direction of the ether’s movement (the same direction as the earth). Light moving in the same direction as the ether should travel faster than light moving against the ether’s motion. They based this assumption on the behavior of other waves—the speed of waves traveling in water, for example, is related to the speed at which the water is flowing.

The Michelson-Morley experiment separated a beam of light with a special piece of glass, half of which was a mirror, called a beam splitter. Half of the beam of light traveled in the direction of the earth’s motion, and half was reflected by the mirror and traveled perpendicular to the earth’s motion. Each beam bounced off other mirrors and came back to the beam splitter, where the split beams were recombined into one beam. The length of the path that both beams of light traveled was exactly the same. In the original beam, the crests and troughs of the light waves were all lined up, but if one of the split beams had covered the distance more slowly than the other, the crests and troughs would no longer be lined up when the beam was recombined. If the crests and troughs were not lined up, the two beams would interfere with one another, producing a distinctive pattern of light and dark bands. If the beams traveled at the same speed, there would be no interference pattern. Michelson and Morley found no interference pattern, even after rotating their experiment through many orientations and performing the experiment at different times of year. The lack of interference demonstrated that the split beams traveled at the same speed and showed that the speed of light was independent of the motion of the earth. This finding indicated that the ether did not exist.

In 1905 Albert Einstein advanced his theory of relativity, in which the speed of light is a universal constant—the same in all directions. In Einstein’s theory, the existence of the ether is impossible. In today's physics, Einstein's special theory of relativity is completely accepted, and the ether is viewed only as a historical relic that does not actually exist.

Coal

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Coal, a combustible organic rock composed primarily of carbon, hydrogen, and oxygen. Coal is burned to produce energy and is used to manufacture steel. It is also an important source of chemicals used to make medicine, fertilizers, pesticides, and other products. Coal comes from ancient plants buried over millions of years in Earth’s crust, its outermost layer. Coal, petroleum, natural gas, and oil shale are all known as fossil fuels because they come from the remains of ancient life buried deep in the crust.

Coal is rich in hydrocarbons (compounds made up of the elements hydrogen and carbon). All life forms contain hydrocarbons, and in general, material that contains hydrocarbons is called organic material. Coal originally formed from ancient plants that died, decomposed, and were buried under layers of sediment during the Carboniferous Period, about 360 million to 290 million years ago. As more and more layers of sediment formed over this decomposed plant material, the overburden exerted increasing heat and weight on the organic matter. Over millions of years, these physical conditions caused coal to form from the carbon, hydrogen, oxygen, nitrogen, sulfur, and inorganic mineral compounds in the plant matter. The coal formed in layers known as seams.

Plant matter changes into coal in stages. In each successive stage, higher pressure and heat from the accumulating overburden increase the carbon content of the plant matter and drive out more of its moisture content. Scientists classify coal according to its fixed carbon content, or the amount of carbon the coal produces when heated under controlled conditions. Higher grades of coal have a higher fixed carbon content.

Cobaltite

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Cobaltite, mineral, a compound of cobalt, arsenic, and sulfur. It occurs in isometric crystals resembling those of pyrite. It is silver white to red in color and has a metallic luster. The specific gravity of cobaltite is between 6 and 6.4, and the hardness is 5.5. Cobaltite occurs in high-temperature vein deposits or in disseminations in rocks associated with nickel and other cobalt minerals. It is mined as an important ore of cobalt. Large deposits are found in Sweden, Norway, Myanmar (formerly known as Burma), Australia, and Ontario in Canada.

Color

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Color, physical phenomenon of light or visual perception associated with the various wavelengths in the visible portion of the electromagnetic spectrum (see Electromagnetic Radiation; Spectrum). As a sensation experienced by humans and some animals, perception of color is a complex neurophysiological process. The methods used for color specification today belong to a technique known as colorimetry and consist of accurate scientific measurements based on the wavelengths of three primary colors.

PRIMARY COLORS

The human eye does not function like a machine for spectral analysis, and the same color sensation can be produced by different physical stimuli. Thus a mixture of red and green light of the proper intensities appears exactly the same as spectral yellow, although it does not contain light of the wavelengths corresponding to yellow. Any color sensation can be duplicated by mixing varying quantities of red, blue, and green. These colors, therefore, are known as the additive primary colors. If light of these primary colors is added together in equal intensities, the sensation of white light is produced. A number of pairs of pure spectral colors called complementary colors also exist; if mixed additively, these will produce the same sensation as white light. Among these pairs are certain yellows and blues, greens and blues, reds and greens, and greens and violets.

Most colors seen in ordinary experience are caused by the partial absorption of white light. The pigments that give color to most objects absorb certain wavelengths of white light and reflect or transmit others, producing the color sensation of the unabsorbed light.

The colors that absorb light of the additive primary colors are called subtractive primary colors. They are magenta (purplish-pink), which absorbs green; yellow, which absorbs blue; and cyan (light greenish-blue), which absorbs red. Thus, if a green light is thrown on a magenta pigment, the eye will perceive black. These subtractive primary colors are also called the pigment primaries. They can be mixed together in varying amounts to match almost any hue. If all three are mixed in about equal amounts, they will produce black. An example of the mixing of subtractive primaries is in color photography and in the printing of colored pictures in magazines, where magenta, yellow, black, and cyan inks are used successively to create natural color. Edwin Herbert Land, an American physicist and inventor of the Polaroid Land camera, demonstrated that color vision depends on a balance between the longer and shorter wavelengths of light. He photographed the same scene on two pieces of black-and-white film, one under red illumination, for long wavelengths, and one under green illumination, for short wavelengths. When both transparencies were projected on the same screen, with a red light in one projector and a green light in the other, a full-color reproduction appeared. The same phenomenon occurred when white light was used in one of the projectors. Reversing the colored lights in the projectors made the scene appear in complementary colors.

Columbite

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Columbite, mineral oxide of niobium, tantalum, iron, and manganese with varying proportions of niobium and tantalum. When the proportion of tantalum exceeds that of niobium, it is called tantalite. Columbite is the principal commercial source of tantalum and niobium. It has a hardness of 6 and a specific gravity that varies from about 5 to about 8 depending on the composition. It is black to red in color, often iridescent, and is usually found in the form of orthorhombic crystals in granite and granitic pegmatite. Columbite deposits are found in Australia, Canada, Greenland, Norway, the former Union of Soviet Socialist Republics, and in several states of the United States.

Combustion

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Combustion, process of rapid oxidation or burning of a substance with simultaneous evolution of heat and, usually, light. In the case of common fuels, the process is one of chemical combination with atmospheric oxygen to produce as the principal products carbon dioxide, carbon monoxide, and water, together with products such as sulfur dioxide that may be generated by the minor constituents of the fuel (see Chemical Reaction). The term combustion, however, also embraces oxidation in the broad chemical sense, and the oxidizing agent may be nitric acid, certain perchlorates, or even chlorine or fluorine. See separate articles on most of the fuels and chemicals mentioned in this article.

Composite Material

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Composite Material, substance that is made up of a combination of two or more different materials. A composite material can provide superior and unique mechanical and physical properties because it combines the most desirable properties of its constituents while suppressing their least desirable properties. For example, a glass-fiber reinforced plastic combines the high strength of thin glass fibers with the ductility and chemical resistance of plastic; the brittleness that the glass fibers have when isolated is not a characteristic of the composite. The opportunity to develop superior products for aerospace, automotive, and recreational applications has sustained the interest in advanced composites. Currently composites are being considered on a broader basis—for applications that include civil engineering structures such as bridges and freeway pillar reinforcement; and for biomedical products, such as prosthetic devices.

Composite materials usually consist of synthetic fibers embedded within a matrix, a material that surrounds and is tightly bound to the fibers. The most widely used type of composite material is polymer matrix composites (PMCs). PMCs consist of fibers made of a ceramic material such as carbon or glass embedded in a plastic matrix. Typically, the fibers make up about 60 percent of a polymer matrix composite by volume. Metal matrices or ceramic matrices can be substituted for the plastic matrix to provide more specialized composite systems called metal matrix composites (MMCs) and ceramic matrix composites (CMCs), respectively.

Critical Point

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Critical Point, in physics, point on the temperature or pressure scale, which marks a change in the physical state of a substance. The critical point of a metal alloy is the temperature during the cooling of the substance at which a molecular rearrangement takes place, giving rise to a different form of the substance, usually with the absorption or evolution of heat. The critical temperature of a gas is the maximum temperature at which the gas can be liquefied; the critical pressure is the pressure necessary to liquefy the gas at the critical temperature. Some gases, such as helium, hydrogen, and nitrogen, have low critical temperatures and require intensive cooling before they can be liquefied. Others, such as ammonia and chlorine, have high critical temperatures and can be liquefied at ordinary room temperature by pressure alone. The accompanying table shows critical temperatures and pressures for representative gases.

A third description of the critical point is the critical volume. This is the volume that one mole of gas would occupy at its critical temperature and pressure. These three quantities: critical temperature, pressure, and volume are called, collectively, the critical constants of a substance.

Decomposition

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Decomposition, in chemistry, the breaking down of a substance or compound, through a chemical reaction, into its simpler components. Such reduction may yield either elements or compounds as products. A common agent of decomposition in chemistry is heat, which can reduce both inorganic and organic compounds to their constituents. Water, for example, decomposes into hydrogen and oxygen when exposed to an electric current. Also, chemical action, as by the use of acids (see Acids and Bases) or alkalies and as accelerated by catalysis, is used in laboratories to reduce compounds. Decomposition is also caused by bacteria, enzymes, and light. Fermentation, for example, occurs because of enzyme actions.

The term decomposition is also applied to the phenomenon of biological decay, or putrefaction, caused by microorganisms. Natural decomposition can also, however, yield useful products, such as petroleum.

Dielectric

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Dielectric, or insulator, substance that is a poor conductor of electricity and that will sustain the force of an electric field passing through it. This property is not exhibited by conducting substances. Two oppositely charged bodies placed on either side of a piece of glass (a dielectric) will attract each other, but if a sheet of copper is instead interposed between the two bodies, the charge will be conducted by the copper.

In most instances the properties of a dielectric are caused by the polarization of the substance. When the dielectric is placed in an electric field, the electrons and protons of its constituent atoms reorient themselves, and in some cases molecules become similarly polarized. As a result of this polarization, the dielectric is under stress, and it stores energy that becomes available when the electric field is removed. The polarization of a dielectric resembles the polarization that takes place when a piece of iron is magnetized. As in the case of a magnet, a certain amount of polarization remains when the polarizing force is removed. A dielectric composed of a wax disk that has hardened while under electric stress will retain its polarization for years. Such dielectrics are known as electrets.

The effectiveness of dielectrics is measured by their relative ability, compared to a vacuum, to store energy, and is expressed in terms of a dielectric constant, with the value for a vacuum taken as unity. The values of this constant for usable dielectrics vary from slightly more than 1 for air up to 100 or more for certain ceramics containing titanium oxide. Glass, mica, porcelain, and mineral oils, often used as dielectrics, have constants ranging from about 2 to 9. The ability of a dielectric to withstand electric fields without losing insulating properties is known as its dielectric strength. A good dielectric must return a large percentage of the energy stores in it when the field is reversed. The fraction lost through so-called electric friction is called the power factor of the dielectric. Dielectrics, particularly those with high dielectric constants, are used extensively in all branches of electrical engineering, where they are employed to increase the efficiency of capacitors. See Capacitor; Electricity; Insulation.

Diffraction

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Diffraction, property of wave motion, in which waves spread and bend as they pass through small openings or around barriers. Diffraction is more pronounced when the opening, or aperture, or the barrier is similar in size to or smaller than the wavelength of the incoming wave. Diffraction is a property of the motion of all waves. For example, if a radio is turned on in one room, the sound from the radio can be heard in an adjacent room even from around a doorway. Similarly, whenever water waves pass an object on the surface of the water, such as a jetty or boat dock, waves that pass the object's edge spread out into the region behind the object and directly blocked by it.

To understand this effect, Dutch physicist Christiaan Huygens proposed that each point of a wave on a flat wave front, or crest, acts like a source of secondary, spherical wavelets, or smaller waves. Before reaching a barrier, these secondary wavelets add to the original wave front. When the wave front approaches an aperture or barrier, only the wavelets approaching the unobstructed region can get past the barrier. When the size of the opening or barrier is large compared with the wavelength of the incoming wave, the sum of the wavelets passing through the aperture is nearly flat. The resulting wave front resembles the original wave front, and little bending occurs. However, when the size of the opening is comparable to or smaller than the wavelength of the incoming wave, it appears as though only a few wavelets can get through. These remaining wavelets are then a source of more wavelets that expand in all directions, and the shape of the new wave front is curved. The wavelets of these diffracted, or bent, waves can now travel different paths and subsequently interfere with each other, producing interference patterns. The shape of these patterns depends on the wavelength and the size of the aperture or barrier. According to Huygens's principle, diffraction can be thought of as the interference of a large number of coherent wave sources. Consequently, diffraction and interference are essentially the same phenomenon.

Rectification

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Rectification (electricity), process of converting an alternating current (AC), which flows back and forth in a circuit, to direct current (DC), which flows only in one direction. A device known as a rectifier, which permits current to pass in only one direction, effectively blocking its flow in the other direction, is inserted into the circuit for the purpose.

Rectification is carried out at all levels of electrical power, from a thousandth of a watt to detect an AM radio signal, to thousands of kilowatts to operate heavy electrical machinery. The first commercial rectifiers were used in the conversion of alternating to direct current in the operation of electrical motors; these early rectifiers were called mechanical commutators. Today, most rectification is carried out by electronic devices, such as combinations of vacuum-tube diodes, and mercury-arc rectifiers.

Most mechanical rectifiers consist of a rotary switch that is synchronized with the current; the switch is arranged to conduct the current in one direction only. Mechanical rectifiers can be designed and constructed to handle heavy currents (up to thousands of amperes) at levels of several thousand volts, and they are still used in heavy electrical machinery.

Electronic rectifiers conduct current in one direction only by the motion of electrical charges inside the device; they can carry currents as high as 500 amp and withstand voltages up to 1000 V without damage. These rectifiers, therefore, can compete with mechanical rectifiers in many power applications. In low-voltage applications, such as in electronic equipment, either vacuum-tube or semiconductor rectifiers are used almost exclusively.

Photoelectric Effect

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Photoelectric Effect, formation and liberation of electrically charged particles in matter when it is irradiated by light or other electromagnetic radiation. The term photoelectric effect designates several types of related interactions. In the external photoelectric effect, electrons are liberated from the surface of a metallic conductor by absorbing energy from light shining on the metal's surface. The effect is applied in the photoelectric cell, in which the electrons liberated from one pole of the cell, the photocathode, migrate to the other pole, the anode, under the influence of an electric field.

Study of the external photoelectric effect played an important role in the development of modern physics. Experiments beginning in 1887 showed that the external photoelectric effect had certain qualities that could not be explained by the theories of that time, in which light and all other electromagnetic radiation was considered to behave like waves. For example, as the light shining on a metal becomes increasingly intense, the classical wave theory of light suggests that the electrons that absorb the light will be liberated from the metal with more and more energy. However, experiments showed that the maximum possible energy of the ejected electrons depends only on the frequency of the incident light, and is independent of the light's intensity.

Photoelectric Cell

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Photoelectric Cell, also phototube, electron tube in which the electrons initiating an electric current originate by photoelectric emission. In its simplest form the phototube is composed of a cathode, coated with a photosensitive material, and an anode. Light falling upon the cathode causes the liberation of electrons, which are then attracted to the positively charged anode, resulting in a flow of current proportional to the intensity of the irradiation. Phototubes may be highly evacuated or may be filled with an inert gas at low pressure to achieve greater sensitivity. In a modification called the multiplier phototube, or the photomultiplier, a series of metal plates are so shaped and arranged that the photoelectric emission is amplified by secondary electron emission. The multiplier phototube is capable of detecting radiation of extremely low intensity; hence, it is an essential tool for those working in the area of nuclear research.

The photoelectric cell, popularly known as the electric eye, is employed in operating burglar alarms, traffic-light controls, and door openers. A phototube and a beam of light (which may be infrared or invisible to the eye) form an essential part of such an electric circuit. The light produced by a bulb at one end of the circuit falls on the phototube located some distance away. Interrupting the beam of light breaks the circuit. This in turn causes a relay to close, which energizes the burglar-alarm, or other, circuit. Various types of phototubes are used in sound recording, television, and the scintillation counter (see Particle Detectors).

Center of Mass

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Center of Mass, that point at which the entire mass of an object may be considered to be located for purposes of understanding the object's motion. The center of mass of a uniform sphere is the point at the center of the sphere; the center of mass of a uniform rod with a circular cross-section is the point at the center of the cross-sectional slice of the rod that is located at the middle of the rod lengthwise. In some irregularly shaped objects, the center of mass may lie outside the object.

When trying to understand and calculate the motion of an object, focusing attention on the center of mass often simplifies the problem. For example, a rod thrown into the air moves in a complicated manner; the rod moves through the air, and at the same time it tends to rotate. If the motion of a point at the tip of the rod were tracked, the path that point would follow would be very complicated. But if the motion of the rod's center of mass were tracked, the point would follow a parabolic path that can easily be described mathematically. Additionally, the complicated rotation of the rod can be described as simple rotational motion about the center of mass. The center of mass can also be useful when examining the motions of complicated systems that are composed of many objects or particles, such as the motion of the planets around the sun.

Phagocytosis

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Phagocytosis (Greek -phagos, “one that eats”; kytos, “cell”), process of ingestion of matter by cells known, in this context, as phagocytes. Single-celled life forms that bodily engulf and ingest foreign matter—whether other cells, bacteria, or nonliving material—display phagocytosis. In multicellular organisms the process is relegated to specialized cells, generally for the purpose of defending the organism as a whole from potentially harmful invaders.

In humans and other higher animals, phagocytes are wandering cells that occur throughout the body. Larger phagocytes, called macrophages, are particularly important in the lymph system, liver, and spleen; amoeboid macrophages also travel throughout the body's tissues, feeding on bacteria and other foreign matter. Smaller phagocytes, which are known as granular leukocytes—a type of white blood cell—are carried throughout the body by the bloodstream. Attracted to sites of infection by chemicals which are emitted by the invading bacteria, they can pass through blood-vessel walls to reach the invaders. The successfulness of the process is related to the nature of the alien material. Proteins in the blood normally coat foreign particles, attracting the phagocytes to adhere and feed. If more-active bacterial forms invade the body, however, they may not be ingested until physically trapped or until coated by particular proteins called antibodies. If still uningested, they may actually be spread throughout the body by the phagocytes.

Glucose

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Glucose, monosaccharide sugar, is found in honey and the juices of many fruits; the alternate name grape sugar is derived from the presence of glucose in grapes. It is the sugar most often produced by hydrolysis of natural glycosides. Glucose is a normal constituent of the blood of animals (see Sugar Metabolism).

Glucose is a white crystalline solid, less sweet than ordinary table sugar. Solutions of glucose rotate the plane of polarization of polarized light to the right; hence the alternative name dextrose (Latin dexter, “right”). Glucose crystallizes in three different forms. The degree of rotation of polarized light is different for each form.

Glucose is formed by the hydrolysis of many carbohydrates, including sucrose, maltose, cellulose, starch, and glycogen. Fermentation of glucose by yeast produces ethyl alcohol and carbon dioxide. Glucose is made industrially by the hydrolysis of starch under the influence of dilute acid or, more commonly, under that of enzymes. It is chiefly used as a sweetening agent in the food-processing industries. It is also used in tanning, in dye baths, in making tableted products, and in medicine for treating dehydration and for intravenous feeding.

Selenium

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Selenium (Greek selēnē, “moon”), symbol Se, semimetallic element with an atomic number of 34. Selenium is in group 16 (or VIa) of the periodic table.

Selenium was discovered in 1817 by the Swedish chemist Baron Jöns Jakob Berzelius in a sulfuric acid residue. It was so called because it was found in association with tellurium (Latin tellus, “earth”).

PROPERTIES AND OCCURRENCE

Chemically, selenium closely resembles sulfur and is related to tellurium. Like sulfur, it exists in several allotropic (distinctly different) forms: a brick-red powder; a brownish-black, glassy, amorphous mass called vitreous selenium; red monoclinic crystals of specific gravity 4.5; and gray, lustrous crystals called gray selenium. It forms selenious acid and selenic acid, the respective salts of which are called selenites and selenates. Gray selenium melts at 217°C (423°F), boils at about 685°C (about 1265°F), and has a specific gravity of 4.81. The atomic weight of selenium is 78.96.

The element occurs in a few selenide minerals, the most common of which is clausthalite, or lead selenide. It also occurs with free sulfur and in many sulfide ores; it is generally obtained as a by-product in the refining of copper-sulfide ores. The yield from by-product sources, however, is insufficient to supply the rapidly increasing industrial demand for the element.

USES

Gray selenium conducts electricity; it is a better conductor of electricity in light than in darkness, the conductivity varying directly with the intensity of light. It is therefore used in many photoelectric devices. In the form of red selenium or as sodium selenide the element is used to impart a scarlet red color to clear glass, glazes, and enamels. It is also used to a great extent as a decolorizer of glass because it neutralizes the greenish tint produced by iron (ferrous) compounds. Small amounts of selenium are added to vulcanized rubber to increase its resistance to abrasion. Sodium selenate is an insecticide used to combat insects that attack cultivated plants, particularly chrysanthemums and carnations; the insecticide is scattered around the roots and is carried by the sap throughout the plant. Selenium sulfide is used in the treatment of dandruff, acne, eczema, seborrheic dermatitis, and other skin diseases.

Semiconductor

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Semiconductor, solid or liquid material, able to conduct electricity at room temperature more readily than an insulator, but less easily than a metal. Electrical conductivity, which is the ability to conduct electrical current under the application of a voltage, has one of the widest ranges of values of any physical property of matter. Such metals as copper, silver, and aluminum are excellent conductors, but such insulators as diamond and glass are very poor conductors (see Electrical Conductor; Insulation; Metals). At low temperatures, pure semiconductors behave like insulators. Under higher temperatures or light or with the addition of impurities, however, the conductivity of semiconductors can be increased dramatically, reaching levels that may approach those of metals. The physical properties of semiconductors are studied in solid-state physics.

CONDUCTION ELECTRONS AND HOLES

The common semiconductors include chemical elements and compounds such as silicon, germanium; selenium, gallium arsenide, zinc selenide, and lead telluride. The increase in conductivity with temperature, light, or impurities arises from an increase in the number of conduction electrons, which are the carriers of the electrical current (See Electricity; Electron). In a pure, or intrinsic, semiconductor such as silicon, the valence electrons, or outer electrons, of an atom are paired and shared between atoms to make a covalent bond that holds the crystal together (See Chemical Reaction). These valence electrons are not free to carry electrical current. To produce conduction electrons, temperature or light is used to excite the valence electrons out of their bonds, leaving them free to conduct current. Deficiencies, or “holes,” are left behind that contribute to the flow of electricity. (These holes are said to be carriers of positive electricity.) This is the physical origin of the increase in the electrical conductivity of semiconductors with temperature. The energy required to excite the electron and hole is called the energy gap.

See also: Doping

Semiconductor

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Doping

Another method to produce free carriers of electricity is to add impurities to, or to “dope,” the semiconductor. The difference in the number of valence electrons between the doping material, or dopant (either donors or acceptors of electrons), and host gives rise to negative (n-type) or positive (p-type) carriers of electricity. This concept is illustrated in the accompanying diagram of a doped silicon (Si) crystal. Each silicon atom has four valence electrons (represented by dots); two are required to form a covalent bond. In n- type silicon, atoms such as phosphorus (P) with five valence electrons replace some silicon and provide extra negative electrons. In p-type silicon, atoms with three valence electrons such as aluminum (Al) lead to a deficiency of electrons, or to holes, which act as positive electrons. The extra electrons or holes can conduct electricity.

When p-type and n-type semiconductor regions are adjacent to each other, they form a semiconductor diode, and the region of contact is called a p-n junction. (A diode is a two-terminal device that has a high resistance to electric current in one direction but a low resistance in the other direction.) The conductance properties of the p-n junction depend on the direction of the voltage, which can, in turn, be used to control the electrical nature of the device. Series of such junctions are used to make transistors and other semiconductor devices such as solar cells, p-n junction lasers, rectifiers, and many others. See Electronics; Laser; Rectification; Solar Energy; Transistor.

Semiconductor devices have many varied applications in electrical engineering. Recent engineering developments have yielded small semiconductor chips containing hundreds of thousands of transistors. These chips have made possible great miniaturization of electronic devices. More efficient use of such chips has been developed through what is called complementary metal-oxide semiconductor circuitry, or CMOS, which consists of pairs of p- and n-channel transistors controlled by a single circuit. In addition, extremely small devices are being made using the technique of molecular-beam epitaxy.

See also Computer; Integrated Circuit; Microprocessor.