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. Actinide Series, a series of 14 radioactive elements in the periodic table with atomic numbers 89 through 102. Only the first four elements in the series have been found in nature in appreciable amounts; the remainder have been produced synthetically. Those elements with atomic numbers of 93 and above are called transuranium elements . The elements constituting the actinide series are, in order of increasing atomic number, actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, and nobelium.

. Actinium, symbol Ac, radioactive metallic element found in all uranium ores. The atomic number of actinium is 89; the element is in the actinide series of the periodic table . Actinium was discovered in 1899 by the French chemist André Louis Debierne. The element is found in uranium ores to the extent of 2 parts to every 10 billion parts of uranium. Two naturally occurring isotopes of actinium are known. Actinium-227 is a member of the actinium series, called the actinium decay series, resulting from the radioactive decay of uranium-235 (see Radioactivity ). It has a half-life of 21.8 years. The other isotope, actinium-228, is a member of the thorium series resulting from the decay of thorium-232. This isotope, known also as mesothorium-2, has a half-life of 6.13 hours. Isotopes ranging in mass number from 209 to 234 are known. Actinium melts at about 1050° C (about 1922° F), boils at about 3200° C (about 5792° F), and has a specific gravity of about 10.

. Radioactivity, spontaneous disintegration of atomic nuclei by the emission of subatomic particles called alpha particles and beta particles , or of electromagnetic rays called X rays and gamma rays. The phenomenon was discovered in 1896 by the French physicist Antoine Henri Becquerel when he observed that the element uranium can blacken a photographic plate, although separated from it by glass or black paper. He also observed that the rays that produce the darkening are capable of discharging an electroscope , indicating that the rays possess an electric charge. In 1898 the French chemists Marie Curie and Pierre Curie deduced that radioactivity is a phenomenon associated with atoms, independent of their physical or chemical state. They also deduced that because the uranium-containing ore pitchblende is more intensely radioactive than the uranium salts that were used by Becquerel, other radioactive elements must be in the ore. They carried through a series of chemical treatments...

. Americium, symbol Am, artificially created, malleable, radioactive metallic element somewhat similar to lead. The atomic number of americium is 95; the element is one of the transuranium elements in the actinide series of the periodic table . Americium was the fourth transuranic element to be synthesized. It was discovered in 1944 and 1945 by the American physicist Glenn Seaborg and his associates at the University of Chicago. They synthesized the americium isotope of mass number 241 by bombarding plutonium-239 with neutrons. Americium isotopes with mass numbers 237 to 247 have been formed; they are all radioactive, with half-lives of from 0.9 minute (americium-232) to about 7400 years (americium-243) (see Radioactivity ). Americium-243 is used as target material in nuclear reactors or particle accelerators for the production of even heavier synthetic elements. Americium melts at about 994° C (about 1821° F) and has a specific gravity of about 14.

. Argon, symbol Ar, inert gaseous element that is the third most prevalent gas in the earth's atmosphere. In group 18 (or VIIIa) of the periodic table , argon is one of the noble gases . The atomic number of argon is 18. Argon was discovered in 1894 by the British scientists Sir William Ramsay and Baron John William Strutt Rayleigh . They were led to this discovery by a discrepancy between the density of supposedly pure nitrogen, prepared from air, and actually pure nitrogen, prepared from ammonium nitrate. Argon is composed of monatomic molecules and is colorless and odorless.It constitutes 0.93 percent of the atmosphere. Argon melts at -189.3° C (-308.2° F) and boils at -185.86° C (-302.55° F). The atomic weight of argon is 39.948. In recent years chemists have been able to force most of the noble gases to form compounds. Researchers at the University of Helsinki achieved this feat with argon in 2000. They created argon fluorohydride (HArF) by shining ultraviolet light on frozen...

. Arsenic, symbol As, extremely poisonous semimetallic element. The atomic number of arsenic is 33. Arsenic is in group 15 (or Va) of the periodic table . Chemically, arsenic is intermediate between metals and nonmetals. Its properties lie, in general, in the middle of the series formed by the family of the elements nitrogen , phosphorus , arsenic, antimony , and bismuth. Arsenic ranks about 52nd in natural abundance among the elements in crustal rocks. When arsenic is heated, it sublimes, passing directly from solid to gaseous form at 613° C (1135° F). A common form of arsenic is gray, metallic in appearance, and has a specific gravity of 5.7. A yellow, nonmetallic form also exists and has a specific gravity of 2.0. The atomic weight of arsenic is 74.9216. Arsenic is used in large quantities in the manufacture of glass to eliminate a green color caused by impurities of iron compounds. A typical charge in a glass furnace contains 0.5 percent of arsenic trioxide. Arsenic is sometimes a...

. Astatine (Greek astatos, “unstable”), symbol At, radioactive element that is the heaviest of the halogens . The atomic number of astatine is 85. Originally called alabamine because of early research with the element at Alabama Polytechnic Institute, it was prepared in 1940 by bombarding bismuth with high-energy alpha particles . The first isotope synthesized had an atomic weight of 211 and a half-life of 7.2 hours. Subsequently, astatine-210 was produced and found to have a half-life of about 8.3 hours. Isotopes of astatine with mass numbers from 200 to 219 have been cataloged, some with half-lives measured in fractions of a second (see Radioactivity ). Astatine is the halogen that behaves most like a metal and that has only radioactive isotopes. It is highly carcinogenic: Mammary and pituitary tumors have been induced in laboratory animals by a single injection. See also Periodic Law .

. Berkelium, symbol Bk, artificially created radioactive metallic element. The atomic number of berkelium is 97; the element is one of the transuranium elements in the actinide series of the periodic table . Berkelium was discovered in 1949 by the American chemists Glenn T. Seaborg , Stanley G. Thompson, and Albert Ghiorso at the University of California laboratories in Berkeley, California, for which the element was named. An isotope of mass number 243 with a half-life of 4.6 hours was produced by bombarding americium-241 with alpha particles accelerated in a cyclotron (see Particle Accelerators ). Subsequently, nine more isotopes were produced, bringing the total range of mass numbers from 242 to 251. The most stable isotope of berkelium, with a half-life of about 1400 years, has a mass number of 247. See also Radioactivity .

. Bohrium, symbol Bh, chemical element with atomic number 107. It is produced artificially by nuclear fusion (in which an element with larger atoms is produced by fusing together smaller atoms of other elements ). Each bohrium atom has a very large nucleus, or central mass, containing positively charged particles called protons and neutral particles called neutrons . The large number of particles in the nucleus makes the atom unstable and causes the atom to split apart into smaller components soon after it is created. The International Union of Pure and Applied Chemistry named element 107 bohrium (Bh), which was previously called unnilseptium, to honor Danish physicist and Nobel laureate Niels Bohr , who made important contributions to nuclear physics and the understanding of atomic structure. Bohrium has the atomic number 107, which means that each bohrium atom contains 107 protons in the nucleus. Scientists have created several isotopes of bohrium, or forms of the element that con...

. Boron, symbol B, hard, brittle semimetallic element with an atomic number of 5. Boron is in group 13 (or IIIa) of the periodic table . Compounds of boron, notably borax, have been known since early times, but the pure element was first prepared in 1808 by the French chemists Joseph Gay-Lussac and Baron Louis Thénard, and independently by the British chemist Sir Humphry Davy . It is a trace element needed for plant growth, but toxic in excess. Research suggests that it is also nutritionally important for bone health in humans and other vertebrates. Pure boron, as usually prepared, is an amorphous powder. A crystalline form can be prepared, however, by dissolving boron in molten aluminum and cooling slowly. The atomic weight of boron is 10.81; the element melts at about 2180°C (about 3956°F), boils at about 3650°C (about 6602°F), and has a specific gravity of 2.35. Boron does not react with water or hydrochloric acid and is unaffected by air at ordinary temperatures. At red heat it co...

. Bromine, symbol Br, poisonous element that at room temperature is a dark, reddish-brown liquid. In group 17 (or VIIa) of the periodic table , bromine is one of the halogens . The atomic number of bromine is 35. Bromine is widely distributed in nature. It melts at -7.25° C (18.95° F), boils at 58.78° C (137.8° F), and has a specific gravity of 3.10; the atomic weight of the element is 79.90. Bromine is so similar in its chemical properties to chlorine , with which it is almost invariably associated, that it was not recognized as a separate element until 1826, when it was discovered and isolated by the French chemist Antoine Jérôme Balard. Bromine is very soluble in a wide variety of organic solvents, such as alcohol , ether , chloroform , and carbon disulfide . It reacts chemically with many compounds and metallic elements and is slightly less active than chlorine. Bromine does not occur in nature as a free element, but is found in bromide compounds. It was formerly a by-product of th...

. Californium, symbol Cf, artificially created radioactive element with an atomic number of 98. Californium is one of the transuranium elements in the actinide series of the periodic table . The isotope of californium with a mass number of 245 was first produced in 1950 at the University of California laboratories in Berkeley by the American chemists Stanley G. Thompson, Kenneth Street, Jr., Albert Ghiroso, and Glenn T. Seaborg . The scientists created californium-245 by bombarding curium -242 with alpha particles in a 152-cm (60-in) cyclotron (see Particle Accelerator ). Californium-245 rapidly decays, with the emission of alpha particles, having a half-life of 44 min. Isotopes, with mass numbers from 240 to 255, were subsequently prepared. Californium-249 is the result of beta decay of berkelium -249. The heavier californium isotopes are produced by neutron bombardment of berkelium-249, which increases the number of protons in the nucleus. Californium-252, with a half-life of 2.6 ...

. Cesium, symbol Cs, white, soft, chemically reactive metallic element. In group 1 (or Ia) of the periodic table , cesium is one of the alkali metals . The atomic number of cesium is 55. Cesium was discovered in 1860 by the German chemist Robert Wilhelm Bunsen and the German physicist Gustav Robert Kirchhoff through the use of a spectroscope (see Spectrum). Cesium ranks about 46th in natural abundance among the elements in crustal rocks. Cesium melts at about 28° C (about 82° F), boils at about 669° C (about 1236° F), and has a specific gravity of 1.88; its atomic weight is 132.905. The natural source yielding the greatest quantity of cesium is the rare mineral pollux (or pollucite). Ores of this mineral found on the island of Elba contain 34 percent of cesium oxide; American ores of pollux, found in Maine and South Dakota, contain 13 percent of the oxide. Cesium also occurs in lepidolite, carnallite , and certain feldspars . It is extracted by separating the cesium compound from the...

. Chlorine, symbol Cl, greenish-yellow gaseous element. In group 17 (or VIIa) of the periodic table , chlorine is one of the halogens . The atomic number of chlorine is 17. Elementary chlorine was first isolated in 1774 by the Swedish chemist Carl Wilhelm Scheele , who thought that the gas was a compound; it was not until 1810 that the British chemist Sir Humphry Davy proved that chlorine was an element and gave it its present name. At ordinary temperatures, chlorine is a greenish-yellow gas that can readily be liquefied under pressure of 5170 torr, or 6.8 atmospheres, at 20° C (68° F). The gas has an irritating odor and in large concentration is dangerous; it was the first substance used as a poison gas in World War I (1914-1919). Free chlorine does not occur in nature, but its compounds are common minerals, and it is the 20th most abundant element in the earth's crust. Chlorine melts at -101° C (-149.8° F), boils at -34.05° C (-29.29° F) at one atmosphere pressure, and has a spe...

. Curium, symbol Cm, radioactive element with an atomic number of 96. Curium is one of the transuranium elements in the actinide series of the periodic table . Curium is radioactively unstable and does not exist in nature. Curium was first produced synthetically by the American chemists Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso in 1944 and was named for Pierre and Marie Curie , research pioneers in radioactivity. The element is made by bombarding the synthetic element plutonium with accelerated particles. Curium is a heavy metal similar in properties to uranium, plutonium, and americium. Curium melts at 1340° C (2444° F) and has a specific gravity estimated at 13.5. Thirteen isotopes, ranging in mass number from 238 to 250, have been discovered; the most stable isotope of curium has an atomic weight of 247. Most isotopes of curium decay by emission of alpha particles ; because alpha radiation is not highly penetrating, curium isotopes, particularly curium-244, can be use...

. Darmstadtium, symbol Ds, chemical element with atomic number 110. It is produced artificially by nuclear fusion (in which an element with larger atoms is produced by fusing together smaller atoms from other elements). Each darmstadtium atom has a very large nucleus, or central mass, containing positively charged particles called protons and neutral particles called neutrons . The large number of particles in the nucleus makes the atom unstable and causes the atom to split apart into smaller components soon after it is created. Darmstadtium was first discovered in 1994 by scientists at the Heavy-Ion Research Laboratory in Darmstadt, Germany. The scientists named darmstadtium after the place of its discovery. Darmstadtium has the atomic number 110, which means that each Ds atom contains 110 protons in the nucleus. Scientists have created a number of isotopes of darmstadtium. Isotopes are forms of the element that contain different numbers of neutrons in the nucleus. For example, Ds-...

. Dubnium, symbol Db, chemical element with atomic number 105, produced artificially by nuclear fusion (in which an element with larger atoms is produced by fusing together two smaller atoms from other elements). Each dubnium atom has a very large nucleus, or central mass, containing positively charged particles called protons and neutral particles called neutrons . The large number of particles in the nucleus makes the atom unstable and causes the atom to split apart into smaller components soon after it is created. To honor the Joint Institute of Nuclear Research (JINR) located in Dubna, Russia, where element 105 was first created in 1970, the International Union of Pure and Applied Chemistry officially named this element dubnium. Element 105 was previously called hahnium, after German physical chemist Otto Hahn, a pioneer in the field of nuclear fission. Dubnium has the atomic number 105, which means that each Db atom contains 105 protons. Scientists have created several isotopes ...

. Dysprosium, symbol Dy, metallic element with an atomic number of 66. Dysprosium is one of the rare earth elements in the lanthanide series of the periodic table . The element was discovered in 1886 by Paul Émile Lecoq de Boisbaudran, who separated one of its compounds from an oxide of holmium . Dysprosium is 42nd in abundance among the elements in the earth's crust. The compounds of dysprosium are found in gadolinite, xenotime, euxenite, and fergusonite in Norway, the United States, Brazil, India, and Australia. Its salts are either yellow or yellow-green in color, the most common being a chloride, a nitrate, and a sulfate. The salts of dysprosium have an extremely high magnetic susceptibility. Dysprosium usually occurs as the white oxide dysprosia, with erbium and holmium, two other rare earth elements. Dysprosia is sometimes used in the control rods of nuclear reactors (see Nuclear Energy ). Dysprosium melts at about 1412° C (about 2574° F), boils at about 2567° C (about 465...

. Einsteinium, symbol Es, artificially created radioactive element with an atomic number of 99. Einsteinium is one of the transuranium elements in the actinide series of the Periodic table . Isotopes of einsteinium with mass numbers ranging from 243 to 256 are known. The element, named in honor of German-born American physicist Albert Einstein , was discovered in 1952 in the debris produced by a thermonuclear explosion (see Nuclear Weapons ). The isotope first identified had an atomic mass of 253 and a half-life of 20 days. Subsequently, the most long-lived of all the known einsteinium isotopes, einsteinium-252, was prepared by irradiating plutonium in a nuclear reactor; however, only small amounts are now being produced (see Radioactivity ).

. Erbium, symbol Er, metallic element, whose atomic number is 68. The Swedish chemist Carl Gustav Mosander discovered erbium in 1843. Erbium occurs mostly in the same minerals and in the same areas as dysprosium . One of the rare earth elements , erbium is 43rd in abundance among the elements of the earth's crust. The atomic weight of erbium is 167.26. The element melts at about 1529° C (about 2784° F), boils at about 2868° C (about 5194° F), and has a specific gravity of 9.1. Metallic erbium has a bright silvery luster. Erbium oxide, is a rose-red compound slowly soluble in many mineral acids, forming a series of rose-colored salts, solutions of which have a sweet, astringent taste. Erbium is used in experimental optical amplifiers that amplify light signals sent along fiber-optic cables (see Fiber Optics ).

. Europium, symbol Eu, soft, silvery metallic element that is among the least abundant of the rare earth elements . Europium is in the lanthanide series of the periodic table ; its atomic number is 63. Europium was discovered spectroscopically by the French chemist Eugène Demarçay in 1896. It ranks 50th in order of abundance of the elements in the earth's crust; it occurs in monazite , bastnaesite, and other rare earth minerals, as well as in fission products of uranium , thorium , and plutonium . Europium melts at 822° C (1512° F) boils at about 1527° C (about 2781° F), and has a specific gravity of 5.2. The atomic weight of europium is 151.96. Europium is used as a phosphor activator. The screen of a color-television tube is treated with europium, which, when bombarded with electrons , produces the color red. Because it readily absorbs neutrons , europium is used in the control of nuclear fission in reactors (see Nuclear Energy ).

. Fermium, symbol Fm, artificially created radioactive element with an atomic number of 100. Fermium is one of the transuranium elements in the actinide series of the periodic table . The element was isolated in 1952 from the debris of a hydrogen bomb explosion by the American chemist Albert Ghiorso and coworkers. Subsequently fermium was prepared synthetically in a nuclear reactor by bombarding plutonium with neutrons and in a cyclotron by bombarding uranium -238 with nitrogen ions. Isotopes with mass numbers from 242 to 259 have been produced; fermium-257, the longest-lived of these isotopes, has a half-life of 80 days. The element was named fermium in 1955 in honor of the Italian American nuclear physicist Enrico Fermi . Fermium does not have any industrial applications. See also Radioactivity .

. Francium, symbol Fr, radioactive metallic element that closely resembles cesium in chemical properties. In group 1 (or Ia) of the periodic table , francium is one of the alkali metals . The atomic number of francium is 87. Marguerite Perey of the Curie Laboratory of the Radium Institute of Paris discovered the element in 1939. Francium is produced when the radioactive element actinium disintegrates. Francium is naturally radioactive; its longest-lived isotope , francium-223, or actinium-K, has a half-life of 22 minutes. It emits a beta particle of 1,100,000 electron volts (eV) energy. Isotopes ranging in atomic weights from 204 to 224 are known. Francium is the heaviest of the alkali metals; it is the most electropositive element. All its isotopes are radioactive and short-lived. See also Radioactivity .

. Gadolinium, symbol Gd, silvery white metallic element with an atomic number of 64. Gadolinium is one of the rare earth elements in the lanthanide series of the periodic table . It is named after the Finnish chemist John Gadolin. Gadolinium occurs with other rare earth elements in many minerals, such as samarskite, gadolinite, monazite , and some varieties of Norwegian ytterspar. It is the 41st element in order of abundance in the crust of the earth. Gadolinium melts at about 1313° C (about 2395° F), boils at about 3273° C (about 5923° F), and has a specific gravity of 7.9. The atomic weight of the element is 157.25. Gadolinium oxide was first separated from other rare earth elements by the Swiss chemist Jean de Marignac in 1880. The oxide and many salts of gadolinium have been prepared. Gadolinium oxide is white and the salts are colorless. Because gadolinium has the largest known cross section, or stopping power, for neutrons of any element, it is used as a component of control ro...

. Gallium, symbol Ga, metallic element that remains in the liquid state over a wider range of temperatures than any other element. Gallium is in group 13 (or IIIa) of the periodic table ; its atomic number is 31. Gallium was discovered spectroscopically by the French chemist Paul Émile Lecoq de Boisbaudran in 1875; a year later he isolated the element in its metallic state. Gallium is blue-gray in color as a solid and silvery as a liquid. It is one of the few metals that are liquid near room temperature. It can be supercooled; like water, it expands upon freezing. The element is about 34th in order of abundance in Earth’s crust. Gallium melts at 30°C (86°F), boils at about 2403°C (about 4357°F), and has a specific gravity of 5.9; the atomic weight of the element is 69.72. Gallium occurs in small quantities in some varieties of zinc blende, bauxite, pyrite, magnetite, and kaolin. Gallium resembles aluminum in forming trivalent salts and oxides; it also forms a few monovalent and divalen...

. Germanium, symbol Ge, hard, brittle, grayish-white, crystalline semimetallic element. The atomic number of germanium is 32; it is in group 14 (or IVa) of the periodic table . The Russian chemist Dmitry Mendeleyev predicted the existence and chemical properties of germanium in 1871; because of its position under silicon in the periodic table, he called it ekasilicon. The element was actually discovered in the silver-sulfide ore argyrodite by the German chemist Clemens Alexander Winkler in 1886. Germanium is in the same chemical family as carbon, silicon, tin, and lead, and resembles these elements in forming organic derivatives such as tetraethyl germanium and tetraphenyl germanium. Germanium forms hydrides—germanomethane, or germane; germanoethane; and germanopropane—analogous to those formed by carbon in the methane series. The most important compounds of germanium are the oxide (germanic acid) and the halides. Germanium is separated from other metals by distillation of the tetrach...

. Hafnium, symbol Hf, metallic element that closely resembles zirconium . Hafnium is one of the transition elements of the periodic table ; the atomic number of hafnium is 72. Hafnium was discovered in Copenhagen in 1923 by the Hungarian chemist Georg von Hevesy and the Dutch physicist Dirk Coster. On the basis of a prediction by the Danish physicist Niels Bohr that element 72 would resemble zirconium in structure, they looked for the element in zirconium ores. Hafnium is found in nearly all ores of zirconium and is 45th in order of abundance of the elements in the crust of the earth. It resembles zirconium so closely in chemical properties and crystal structure that separation of the two elements is extremely difficult. Separation is accomplished most efficiently by means of the ion-exchange technique. Hafnium is used in the manufacture of tungsten filaments. Because of its resistance to high temperatures, it is used with zirconium as a structural material in nuclear power plants. ...

. Hassium, symbol Hs, chemical element with atomic number 108. It is produced artificially by nuclear fusion (in which an element with larger atoms is produced by fusing together smaller atoms from other elements). Each hassium atom has a very large nucleus, or central mass, containing positively charged particles called protons and neutral particles called neutrons . The large number of particles in the nucleus makes the atom unstable and causes the atom to split apart into smaller components soon after it is created. In 1997 the International Union of Pure and Applied Chemistry named element 108 hassium (Hs), which was previously called unniloctium, to honor the Heavy-Ion Research Laboratory in Darmstadt, Germany. Researchers at this laboratory discovered elements 107, 108, 109, 111, and 112. The name hassium is derived from the German state Hassia, which is where the research was performed. Hassium has the atomic number 108, which means that each Hs atom contains 108 protons in th...

. Helium (Greek helios, “sun”), symbol He, inert, colorless, odorless gaseous element. In group 18 (or VIIIa) of the periodic table , helium is one of the noble gases . The atomic number of helium is 2. The French astronomer Pierre Janssen discovered helium in the spectrum of the corona of the sun during an eclipse in 1868. Shortly afterward it was identified as an element and named by the British chemist Sir Edward Frankland and the British astronomer Sir Joseph Norman Lockyer . The gas was first isolated from terrestrial sources in 1895 by the British chemist Sir William Ramsay , who discovered it in cleveite, a uranium-bearing mineral. In 1907 the British physicist Sir Ernest Rutherford showed that alpha particles are the nuclei of helium atoms, which later investigation confirmed. Helium has monatomic molecules, and is the lightest of all gases except hydrogen. Helium solidifies at -272.2° C (-457.9° F) at pressures above 19,000 torr (25 atmospheres); helium boils at -268.9° C (-...

. Holmium, symbol Ho, silver-colored metallic element with an atomic number of 67. Holmium is one of the most paramagnetic substances known (see Magnetism ). The element has few practical applications, though it has been used in some electronic devices and as a catalyst in industrial chemical reactions (see Catalysis ). Holmium was discovered in 1878 by the Swiss chemists Jacques Louis Soret and Marc Delafontaine, and, independently, by the Swedish chemist Per Teodor Cleve in 1879. Cleve named the element after his native city of Stockholm, Sweden (the latinized name of Stockholm is Holmia). Holmium is one of the least abundant of the rare earth metals , ranking 55th in order of abundance of the elements in the earth's crust. Holmium has an atomic weight of 164.93. It melts at about 1474° C (about 2685° F), boils at about 2700° C (about 4892° F), and has a specific gravity of 8.8. Holmium occurs in gadolinite and other minerals containing rare earths. Holmium oxide, a grayish-white...

. Indium, symbol In, soft, malleable, silvery white metallic element. The atomic number of indium is 49; the element is in group 13 (or IIIa) of the periodic table . Indium was discovered spectroscopically in 1863 by the German chemists Hieronymus Theodor Richter and Ferdinand Reich. It ranks 63rd in order of abundance of the elements in the surface of the earth. Indium melts at about 157°C (about 315°F), boils at about 2080°C (about 3776°F), and has a specific gravity of 7.3. The atomic weight of indium is 114.82. Indium never occurs as a free metal and is usually found as the sulfide; in certain zinc blendes; and in tungsten, tin, and iron ores. It is used as an alloying agent with nonferrous metals, in bearing alloys , and in nuclear-reactor control rods. Certain indium compounds have unique semiconductor properties.

. Krypton (Greek kryptos, “hidden”), symbol Kr, colorless, odorless gaseous element that makes up a tiny fraction of the earth's atmosphere . In group 18 (or VIIIa) of the periodic table , krypton is one of the noble gases . The atomic number of krypton is 36. Krypton was first isolated in 1898 by the British chemists Sir William Ramsay and Morris William Travers by fractional distillation of a mixture of the noble gases. Krypton is present in the atmosphere to the extent of 1 part in 20 million by volume or 1 part in 7 million by weight. Several compounds of krypton were discovered in 1962 and 1963. Krypton melts at -157.21° C (-250.98° F) and boils at -153.35° C (-244.03° F); liquid krypton has a specific gravity of 2.41 at its boiling point. The atomic weight of krypton is 83.80. Krypton is used alone or with argon and neon in incandescent bulbs. It emits a characteristic bright, orange-red color in an electric-discharge tube; such tubes filled with krypton are used in light...

. Lanthanum (Greek lanthanein, “to escape notice”), symbol La, metallic element with an atomic number of 57. Lanthanum is one of the rare earth elements of the periodic table . Lanthanum is often regarded as the first member of the lanthanide series , to which it gives its name. Lanthanum was discovered by the Swedish chemist Carl Gustav Mosander in 1839. It burns in air at about 450° C (about 842° F) to form lanthanum oxide, La2O3. It forms colorless trivalent salts, including one of the strongest trivalent bases, which is used by analytical chemists. It generally occurs with other rare earth elements in such minerals as apatite and monazite and in certain kinds of calcite and fluorspar . It is fairly common, ranking 28th in order of abundance of the elements in the earth's crust. Impure lanthanum is used in alloys such as misch metal, of which lanthanum is a major constituent. Cigarette-lighter flints are made from this alloy. Lanthanum oxide is used in certain types of optic...

. Lawrencium, symbol Lr, artificially created radioactive metallic element with an atomic number of 103. Lawrencium is one of the transuranium elements of the periodic table . Named in honor of the American physicist Ernest Lawrence , it was discovered in 1961 at the Lawrence Berkeley National Laboratory of the University of California by American chemist Albert Ghiorso and his colleagues. A mixture of californium isotopes was bombarded with boron ions to produce short-lived lawrencium isotopes. Isotopes with mass numbers from 255 to 260 have been prepared. The most stable isotope, with a half-life of about 3 minutes, has a mass number of 260. Only small amounts of lawrencium have been produced. See also Radioactivity .

. Lutetium, symbol Lu, silvery white metallic element with an atomic number of 71. Lutetium is one of the transition elements of the periodic table . Lutetium was discovered independently by two investigators, the French chemist Georges Urbain in 1907 and the Austrian chemist Carl Auer von Welsbach about the same time. It was named by Urbain, who derived the word from Lutetia, the ancient name of Paris. Lutetium occurs in various rare earth minerals, usually associated with yttrium . It is the rarest of the rare earth elements and ranks 59th in order of abundance of the elements in the earth's crust. Several trivalent salts are known. A natural radioactive isotope of lutetium that has a half-life of about 30 billion years is used in determining the age of meteorites in relation to the age of the earth. Lutetium melts at about 1663° C (about 3025° F), boils at about 3402° C (about 6156° F) and has a specific gravity of 9.84. The atomic weight of lutetium is 174.97.

. Meitnerium, symbol Mt, chemical element with atomic number 109. It is produced artificially by nuclear fusion (in which a chemical element with larger atoms is produced by fusing together smaller atoms from other elements). Each meitnerium atom has a very large nucleus, or central mass, containing positively charged particles called protons and neutral particles called neutrons . The large number of particles in the nucleus makes the atom unstable and causes the atom to split apart into smaller components soon after it is created. The International Union of Pure and Applied Chemistry named element 109 meitnerium (Mt), which was previously called unnilennium, to honor Austrian Swedish physicist Lise Meitner , a pioneer in the field of nuclear fission (the splitting of atomic nuclei). Meitnerium has the atomic number 109, which means that each Mt atom contains 109 protons in the nucleus. Scientists have created several isotopes of meitnerium, or forms of the element that contain dif...

. Mendelevium, symbol Md, artificially created radioactive element with an atomic number of 101. Mendelevium is one of the transuranium elements in the actinide series of the periodic table . Named for the Russian chemist Dmitry Mendeleyev , mendelevium-256 was discovered in 1955 at the University of California, Berkeley; it was produced by bombarding einsteinium -253 with alpha particles accelerated in a cyclotron (see Particle Accelerators ). The isotope produced had a half-life of about 1.3 hours. The most stable isotope, mendelevium-258, has a half-life of 54 days. See also Radioactivity .

. Alpha Particle, positively charged nuclear particle, symbol a, consisting of two protons bound to two neutrons. Alpha particles are emitted spontaneously in some types of radioactive decay (see Radioactivity ). They are also produced when helium -4 atoms are completely ionized (see Ion ; see Ionization ).

. Particle Accelerators, in physics, devices used to accelerate charged elementary particles or ions to high energies. Particle accelerators today are some of the largest and most expensive instruments used by physicists. They all have the same three basic parts: a source of elementary particles or ions, a tube pumped to a partial vacuum in which the particles can travel freely, and some means of speeding up the particles. Charged particles can be accelerated by an electrostatic field. For example, by placing electrodes with a large potential difference at each end of an evacuated tube, British scientists John D. Cockcroft and Ernest Thomas Sinton Walton were able to accelerate protons to 250,000 eV (see Electron Volt ). Another electrostatic accelerator is the Van de Graaff accelerator, which was developed in the early 1930s by the American physicist Robert Jemison Van de Graaff. This accelerator uses the same principles as the Van de Graaff Generator . The Van de Graaff accelerat...

. Neodymium, symbol Nd, silvery metallic element with an atomic number of 60. Neodymium is one of the rare earth elements in the lanthanide series of the periodic table . Neodymium was isolated in 1885 by the Austrian chemist Baron Carl Auer von Welsbach, who separated it from praseodymium. Neodymium and praseodymium had previously been regarded as a single element, called didymium. Neodymium ranks 27th in order of abundance of the elements in the earth's crust. It forms trivalent salts, which are rose-red or reddish-violet in color. The metal's oxide, Nd2O3, is used in the glass of color-television tubes to increase contrast, and in lasers . Neodymium melts at about 1021° C (about 1870° F), boils at about 3074° C (about 5565° F), and has a specific gravity of 7.01. The atomic weight of neodymium is 144.24.

. Neon, symbol Ne, colorless, odorless, nonreactive gaseous element that makes up a tiny fraction of Earth's atmosphere . A member of group 18 (or VIIIa) of the periodic table , neon is one of the noble gases . The atomic number of neon—that is, the number of protons in the nucleus of a neon atom—is 10. British chemists Sir William Ramsay and Morris Travers first separated neon from other noble gases in 1898. Neon and the other noble gases do not normally form compounds with other elements. During the past few decades chemists have managed to induce several of the noble gases to form compounds with other extremely reactive elements, such as fluorine, but neon and helium have so far resisted these efforts. Neon melts at -248.59°C (-415.46°F), boils at -246.08°C (-410.94°F), and has a specific gravity of 0.8999 at 0°C (32°F). The atomic weight of neon is 20.18. Stars much more massive than the Sun produce neon during the later stages of nuclear fusion. The abundance of neon on Earth...

. Noble Gases, also inert gases, group of six gaseous chemical elements constituting the group 18 (or VIIIa) of the periodic table . They are, in order of increasing atomic weight, helium , neon , argon , krypton , xenon , and radon . For many years chemists believed that these gases, because their outermost shells were completely filled with electrons, were inert—that is, that they would not enter into chemical combinations with other elements or compounds. This is now known not to be true, at least for the four heaviest inert gases—argon, krypton, xenon, and radon. In 1962, Neil Bartlett, a British chemist working in Canada, succeeded in making the first complex xenon compound. His work was confirmed by scientists at Argonne National Laboratory in Illinois, who made the first simple compound of xenon and fluorine (xenon tetrafluoride) and later made radon and krypton compounds. Although krypton compounds were made with considerable difficulty, both xenon and radon reacted readily wit...

. Neptunium, symbol Np, radioactive metallic element with an atomic number of 93. Neptunium is one of the transuranium elements in the actinide series of the periodic table . Neptunium is a silvery metal that exists in at least three different crystalline forms, hence the variations in specific gravity (from 18 to 20). The element is reactive and shows four ionic oxidation states. It was discovered in 1940 by the American physicists Edwin M. McMillan and Philip H. Abelson. It is produced by bombardment of uranium -238 with neutrons ; the resultant uranium-239 decays radioactively by emitting a beta particle to form neptunium-239. The neptunium isotope in turn emits a beta particle, forming the important isotope plutonium -239, one of the materials of which atomic bombs are made. Isotopes of neptunium with mass numbers from 228 to 242 are known. The most stable, neptunium-237, has a half-life of 2.14 million years. It was discovered by the American chemists Glenn T. Seaborg and Arth...

. Niobium or Columbium, symbol Nb, steel-gray, lustrous, ductile, and malleable metallic element. The atomic number of niobium is 41. Niobium is one of the transition elements of the periodic table . This metal was discovered in 1801 by the British chemist Charles Hatchett. Niobium burns when heated in air and combines with nitrogen, hydrogen, and the halogens. It resists the actions of most acids. Its principal use is as an alloying element in stainless steel, to which it lends additional corrosion resistance, particularly at high temperatures. Niobium ranks about 32nd in natural abundance among the elements in crustal rock. It occurs, associated with the similar element tantalum , in various minerals, the most important of which is called columbite or tantalite, depending on which of the two elements predominates. Pure niobium has excellent characteristics as a construction material in nuclear power plants. Niobium melts at about 2468° C (about 4474° F), boils at about 5127° C (abo...

. Nitrogen, symbol N, gaseous element that makes up the largest portion of the earth's atmosphere . The atomic number of nitrogen is 7. Nitrogen is in group 15 (or Va) of the periodic table . Nitrogen was isolated by the British physician Daniel Rutherford in 1772 and recognized as an elemental gas by the French chemist Antoine Laurent Lavoisier about 1776. Nitrogen is a colorless, odorless, tasteless, nontoxic gas. It can be condensed into a colorless liquid, which can in turn be compressed into a colorless, crystalline solid. Nitrogen exists in two natural isotopic forms, and four radioactive isotopes have been artificially prepared. Nitrogen melts at -210.01° C (-346.02° F), boils at -195.79° C (-320.42° F), and has a density of 1.251 g/liter at 0° C (32° F) and 1 atmosphere pressure. The atomic weight of nitrogen is 14.007. Nitrogen is obtained from the atmosphere by passing air over heated copper or iron. The oxygen is removed from the air, leaving nitrogen mixed with inert ga...

. Nobelium, symbol No, radioactive metallic element with an atomic number of 102. Nobelium is one of the transuranium elements in the actinide series of the periodic table . The element is named for the Swedish inventor and philanthropist Alfred Bernhard Nobel . Nobelium is not found in nature but is produced artificially in the laboratory. Separate discovery of the element was first claimed in 1957 by scientific groups in the United States, Britain, and Sweden, but the first confirmed discovery of a nobelium isotope, by a team of scientists at the Lawrence Berkeley National Laboratory in Berkeley, California, took place in 1958. The isotope was created by bombarding curium isotopes with carbon ions. Chemically, the properties of nobelium are unknown, but because it is an actinide, its properties should somewhat resemble those of the rare earth elements. Isotopes with mass numbers from 250 to 259 and 262 are known. The most stable isotope, nobelium-259, has a half-life of 58 minute...

. Phosphorus, symbol P, reactive nonmetallic element that is important to living organisms and has many industrial uses. The atomic number of phosphorus is 15, and its atomic weight is 30.974. Phosphorus is in group 15 (or Va) of the periodic table . Phosphorus was discovered about 1669 by the German alchemist Hennig Brand in the course of experiments in which he attempted to prepare gold from silver. PROPERTIES AND OCCURRENCE Phosphorus exists in three main allotropic (distinctly different) forms: ordinary (or white) phosphorus, red phosphorus, and black phosphorus. Of these, only white and red phosphorus are of commercial importance. When freshly prepared, ordinary phosphorus is white, but it turns light yellow when exposed to sunlight. It is a crystalline, translucent, waxy solid, which glows faintly in moist air and is extremely poisonous. It ignites spontaneously in air at 34° C (93° F) and must be stored under water. It is insoluble in water, slightly soluble in organic solvents,...

. Phosphoric Acid, common acid of phosphorus that is the source of industrially important compounds called phosphates. At room temperature, phosphoric acid is a crystalline material with a specific gravity of 1.83. The solid melts at 42.35° C (108.23° F). Phosphoric acid is usually stored and sold as a solution. Phosphoric acid is made by treating calcium phosphate rock with sulfuric acid, followed by filtration of the resultant liquid to remove calcium sulfate. It can be prepared also by burning phosphorus vapor and treating the resulting oxide with steam. The acid is useful in the laboratory because of its resistance to oxidation, to reduction, and to evaporation. Among the many uses of phosphoric acid are as an ingredient in soft drinks and dental cements, as a catalyst, in rustproofing metals, and in making phosphates, which are used in water softeners, fertilizers, and detergents. PHOSPHATES Phosphates are products formed by the replacement of some or all of the hydrogen of a pho...