Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Alphabetic list of Chemical Elements (V-W-X-Y-Z)

.

Name: Vanadium
Symbol: V
Atomic Number: 23
Atomic Weight: 50.9415
Group: Transition metals
Date Discovered: 1801
Discovered By: Andrés del Rio (disputed), or Nils Sefström 1830

Name: Xenon
Symbol: Xe
Atomic Number: 54
Atomic Weight: 131.29
Group: Noble gases
Date Discovered: 1898
Discovered By: William Ramsay and Morris Travers

Name: Ytterbium
Symbol: Yb
Atomic Number: 70
Atomic Weight: 173.04
Group: Lanthanide
Date Discovered: series 1878
Discovered By: Jean Charles de Marignac

Name: Yttrium
Symbol: Y
Atomic Number: 39
Atomic Weight: 88.906
Group: Transition metals
Date Discovered: 1794
Discovered By: Johan Gadolin

Name: Zinc
Symbol: Zn
Atomic Number: 30
Atomic Weight: 65.409
Group: Transition metals
Date Discovered: prehistoric
Discovered By: unknown

Name: Zirconium
Symbol: Zr
Atomic Number: 40
Atomic Weight: 91.224
Group: Transition metals
Date Discovered: 1789
Discovered By: Martin Klaproth

Alphabetic list of Chemical Elements (T-U)

.

Name: Tantalum
Symbol: Ta
Atomic Number: 73
Atomic Weight: 180.948
Group: Transition metals
Date Discovered: 1802
Discovered By: Anders Ekeberg

Name: Technetium
Symbol: Tc
Atomic Number: 43
Atomic Weight: (98)
Group: Transition metals
Date Discovered: 1937
Discovered By: Carlo Perrier and Emilio Segrè

Name: Tellurium
Symbol: Te
Atomic Number: 52
Atomic Weight: 127.60
Group: Nonmetals
Date Discovered: 1782
Discovered By: Franz Müller

Name: Terbium
Symbol: Tb
Atomic Number: 65
Atomic Weight: 158.9253
Group: Lanthanide
Date Discovered: series 1843
Discovered By: Carl Mosander

Name: Thallium
Symbol: Tl
Atomic Number: 81
Atomic Weight: 204.3833
Group: Other metals
Date Discovered: 1861
Discovered By: William Crookes (isolated by William Crookes and Claude August Lamy, independently of each other, in 1862

Name: Thorium
Symbol: Th
Atomic Number: 90
Atomic Weight: 232.0381
Group: Actinide
Date Discovered: series 1828
Discovered By: Jöns Berzelius

Name: Thulium
Symbol: Tm
Atomic Number: 69
Atomic Weight: 168.9342
Group: Lanthanide
Date Discovered: series 1879
Discovered By: Per Cleve

Name: Tin
Symbol: Sn
Atomic Number: 50
Atomic Weight: 118.711
Group: Other metals
Date Discovered: prehistoric
Discovered By: unknown

Name: Titanium
Symbol: Ti
Atomic Number: 22
Atomic Weight: 47.867
Group: Transition metals
Date Discovered: 1790
Discovered By: William Gregor

Name: Tungsten
Symbol: W
Atomic Number: 74
Atomic Weight: 183.84
Group: Transition metals
Date Discovered: 1783
Discovered By: isolated by Juan José Elhuyar and Fausto Elhuyar

Name: Ununbium
Symbol: Uub
Atomic Number: 112
Atomic Weight: (277)
Group: Transition metals
Date Discovered: 1996
Discovered By: team at the Heavy-Ion Research Laboratory, Darmstadt, Germany

Name: Ununhexium
Symbol: Uuh
Atomic Number: 116
Atomic Weight: (292)
Group: Other metals
Date Discovered: 2000
Discovered By: team at the Joint Institute for Nuclear Research, Dubna, Russia

Name: Ununquadium
Symbol: Uuq
Atomic Number: 114
Atomic Weight: (285)
Group: Other metals
Date Discovered: 1998
Discovered By: team at the Joint Institute for Nuclear Research, Dubna, Russia

Name: Unununium
Symbol: Uuu
Atomic Number: 111
Atomic Weight: (272)
Group: Transition metals
Date Discovered: 1994
Discovered By: team at the Heavy-Ion Research Laboratory, Darmstadt, Germany

Name: Uranium
Symbol: U
Atomic Number: 92
Atomic Weight: 238.0289
Group: Actinide
Date Discovered: series 1789
Discovered By: Martin Klaproth (isolated by Eugène Péligot 1841)

Alphabetic list of Chemical Elements (Q-R-S)

.

Name: Radium
Symbol: Ra
Atomic Number: 88
Atomic Weight: (226)
Group: Alkaline earth metals
Date Discovered: 1898
Discovered By: Marie Curie

Name: Radon
Symbol: Rn
Atomic Number: 86
Atomic Weight: (222)
Group: Noble gases
Date Discovered: 1900
Discovered By: Friedrich Dorn

Name: Rhenium
Symbol: Re
Atomic Number: 75
Atomic Weight: 186.207
Group: Transition metals
Date Discovered: 1925
Discovered By: Walter Noddack, Ida Tacke, and Otto Berg

Name: Rhodium
Symbol: Rh
Atomic Number: 45
Atomic Weight: 102.9055
Group: Transition metals
Date Discovered: 1804
Discovered By: William Wollaston

Name: Rubidium
Symbol: Rb
Atomic Number: 37
Atomic Weight: 85.4678
Group: Alkali metals
Date Discovered: 1861
Discovered By: Robert Bunsen and Gustav Kirchhoff

Name: Ruthenium
Symbol: Ru
Atomic Number: 44
Atomic Weight: 101.07
Group: Transition metals
Date Discovered: 1827
Discovered By: G. W. Osann (isolated by Karl Klaus 1844)

Name: Rutherfordium
Symbol: Rf
Atomic Number: 104
Atomic Weight: (261)
Group: Transition metals
Date Discovered: 1969
Discovered By: claimed by U.S. scientist Albert Ghiorso and coworkers (disputed by Soviet workers)

Name: Samarium
Symbol: Sm
Atomic Number: 62
Atomic Weight: 150.36
Group: Lanthanide
Date Discovered: series 1879
Discovered By: Paul Lecoq de Boisbaudran

Name: Scandium
Symbol: Sc
Atomic Number: 21
Atomic Weight: 44.9559
Group: Transition metals
Date Discovered: 1876
Discovered By: Lars Nilson

Name: Seaborgium
Symbol: Sg
Atomic Number: 106
Atomic Weight: (266)
Group: Transition metals
Date Discovered: 1974
Discovered By: claimed by Georgii Flerov and coworkers, and independently by Albert Ghiorso and coworkers

Name: Selenium
Symbol: Se
Atomic Number: 34
Atomic Weight: 78.96
Group: Nonmetals
Date Discovered: 1817
Discovered By: Jöns Berzelius

Name: Silicon
Symbol: Si
Atomic Number: 14
Atomic Weight: 28.0855
Group: Nonmetals
Date Discovered: 1823
Discovered By: Johan Arfwedson

Name: Silver
Symbol: Ag
Atomic Number: 47
Atomic Weight: 107.8682
Group: Transition metals
Date Discovered: prehistoric
Discovered By: unknown

Name: Sodium
Symbol: Na
Atomic Number: 11
Atomic Weight: 22.9898
Group: Alkali metals
Date Discovered: 1807
Discovered By: Humphry Davy

Name: Strontium
Symbol: Sr
Atomic Number: 38
Atomic Weight: 87.62
Group: Alkaline earth metals
Date Discovered: 1808
Discovered By: Humphry Davy

Name: Sulfur
Symbol: S
Atomic Number: 16
Atomic Weight: 32.067
Group: Nonmetals
Date Discovered: prehistoric
Discovered By: unknown

Alphabetic list of Chemical Elements (N-O-P)

.

Name: Neodymium
Symbol: Nd
Atomic Number: 60
Atomic Weight: 144.24
Group: Lanthanide
Date Discovered: series 1885
Discovered By: Carl von Welsbach

Name: Neon
Symbol: Ne
Atomic Number: 10
Atomic Weight: 20.1798
Group: Noble gases
Date Discovered: 1898
Discovered By: William Ramsay and Morris Travers

Name: Neptunium
Symbol: Np
Atomic Number: 93
Atomic Weight: (237)
Group: Actinide
Date Discovered: series 1940
Discovered By: Edwin McMillan and Philip Abelson

Name: Nickel
Symbol: Ni
Atomic Number: 28
Atomic Weight: 58.6934
Group: Transition metals
Date Discovered: 1751
Discovered By: Axel Cronstedt

Name: Niobium
Symbol: Nb
Atomic Number: 41
Atomic Weight: 92.9064
Group: Transition metals
Date Discovered: 1801
Discovered By: Charles Hatchett

Name: Nitrogen
Symbol: N
Atomic Number: 7
Atomic Weight: 14.0067
Group: Nonmetals
Date Discovered: 1772
Discovered By: Daniel Rutherford

Name: Nobelium
Symbol: No
Atomic Number: 102
Atomic Weight: (259)
Group: Actinide
Date Discovered: series 1958
Discovered By: Albert Ghiorso, Torbjørn Sikkeland, J. R. Walton, and Glenn Seaborg

Name: Osmium
Symbol: Os
Atomic Number: 76
Atomic Weight: 190.23
Group: Transition metals
Date Discovered: 1804
Discovered By: Smithson Tennant

Name: Oxygen
Symbol: O
Atomic Number: 8
Atomic Weight: 15.9994
Group: Nonmetals
Date Discovered: 1774
Discovered By: Joseph Priestley and Karl Scheele, independently of each other

Name: Palladium
Symbol: Pd
Atomic Number: 46
Atomic Weight: 106.42
Group: Transition metals
Date Discovered: 1804
Discovered By: William Wollaston

Name: Phosphorus
Symbol: P
Atomic Number: 15
Atomic Weight: 30.9738
Group: Nonmetals
Date Discovered: 1674
Discovered By: Hennig Brand

Name: Platinum
Symbol: Pt
Atomic Number: 78
Atomic Weight: 195.08
Group: Transition metals
Date Discovered: 1557
Discovered By: Julius Scaliger

Name: Plutonium
Symbol: Pu
Atomic Number: 94
Atomic Weight: (244)
Group: Actinide
Date Discovered: series 1940
Discovered By: Glenn Seaborg, Edwin McMillan, Joseph Kennedy, and Arthur Wahl

Name: Polonium
Symbol: Po
Atomic Number: 84
Atomic Weight: (209)
Group: Other metals
Date Discovered: 1898
Discovered By: Marie and Pierre Curie

Name: Potassium
Symbol: K
Atomic Number: 19
Atomic Weight: 39.0983
Group: Alkali metals
Date Discovered: 1807
Discovered By: Humphry Davy

Name: Praseodymium
Symbol: Pr
Atomic Number: 59
Atomic Weight: 140.908
Group: Lanthanide
Date Discovered: series 1885
Discovered By: Carl von Welsbach

Name: Promethium
Symbol: Pm
Atomic Number: 61
Atomic Weight: (145)
Group: Lanthanide
Date Discovered: series 1945
Discovered By: J. A. Marinsky, Lawrence

Name: Protactinium
Symbol: Pa
Atomic Number: 91
Atomic Weight: 231.036
Group: Actinide
Date Discovered: series 1913
Discovered By: Kasimir Fajans and O. Göhring

Alphabetic list of Chemical Elements (I-J-K-L-M)

.

Name: Indium
Symbol: In
Atomic Number: 49
Atomic Weight: 114.818
Group: Other metals
Date Discovered: 1863
Discovered By: Ferdinand Reich and Hieronymus Richter

Name: Iodine
Symbol: I
Atomic Number: 53
Atomic Weight: 126.9045
Group: Halogens
Date Discovered: 1811
Discovered By: Bernard Courtois

Name: Iridium
Symbol: Ir
Atomic Number: 77
Atomic Weight: 192.217
Group: Transition metals
Date Discovered: 1804
Discovered By: Smithson Tennant

Name: Iron
Symbol: Fe
Atomic Number: 26
Atomic Weight: 55.845
Group: Transition metals
Date Discovered: prehistoric
Discovered By: unknown

Name: Krypton
Symbol: Kr
Atomic Number: 36
Atomic Weight: 83.798
Group: Noble gases
Date Discovered: 1898
Discovered By: William Ramsay and Morris Travers

Name: Lanthanum
Symbol: La
Atomic Number: 57
Atomic Weight: 138.9055
Group: Lanthanide
Date Discovered: series 1839
Discovered By: Carl Mosander

Name: Lawrencium
Symbol: Lr
Atomic Number: 103
Atomic Weight: (260)
Group: Transition metals
Date Discovered: 1961
Discovered By: Albert Ghiorso, Torbjørn Sikkeland, Almon Larsh, and Robert Latimer

Name: Lead
Symbol: Pb
Atomic Number: 82
Atomic Weight: 207.2
Group: Other metals
Date Discovered: prehistoric
Discovered By: unknown

Name: Lithium
Symbol: Li
Atomic Number: 3
Atomic Weight: 6.941
Group: Alkali metals
Date Discovered: 1817
Discovered By: Johan Arfwedson

Name: Lutetium
Symbol: Lu
Atomic Number: 71
Atomic Weight: 174.967
Group: Transition metals
Date Discovered: 1907
Discovered By: Georges Urbain and Carl von Welsbach, independently of each other

Name: Magnesium
Symbol: Mg
Atomic Number: 12
Atomic Weight: 24.3051
Group: Alkaline earth metals
Date Discovered: 1755
Discovered By: Joseph Black (oxide isolated by Humphry Davy 1808; pure form isolated by Antoine-Alexandre-Brutus Bussy 1828)

Name: Manganese
Symbol: Mn
Atomic Number: 25
Atomic Weight: 54.938
Group: Transition metals
Date Discovered: 1774
Discovered By: Johann Gottlieb Gahn

Name: Meitnerium
Symbol: Mt
Atomic Number: 109
Atomic Weight: (268)
Group: Transition metals
Date Discovered: 1982
Discovered By: Peter Armbruster and coworkers

Name: Mendelevium
Symbol: Md
Atomic Number: 101
Atomic Weight: (258)
Group: Actinide
Date Discovered: series 1955
Discovered By: Albert Ghiorso, Bernard G. Harvey, Gregory Choppin, Stanley Thompson, and Glenn Seaborg

Name: Mercury
Symbol: Hg
Atomic Number: 80
Atomic Weight: 200.59
Group: Transition metals
Date Discovered: prehistoric
Discovered By: unknown

Name: Molybdenum
Symbol: Mo
Atomic Number: 42
Atomic Weight: 95.94
Group: Transition metals
Date Discovered: 1781
Discovered By: named by Karl Scheele (isolated by Peter Jacob Hjelm 1782)

Alphabetic list of Chemical Elements (E-F-G-H)

.

Name: Einsteinium
Symbol: Es
Atomic Number: 99
Atomic Weight: (252)
Group: Actinide
Date Discovered: series 1952
Discovered By: Albert Ghiorso and coworkers

Name: Erbium
Symbol: Er
Atomic Number: 68
Atomic Weight: 167.26
Group: Lanthanide
Date Discovered: series 1843
Discovered By: Carl Mosander

Name: Europium
Symbol: Eu
Atomic Number: 63
Atomic Weight: 151.966
Group: Lanthanide
Date Discovered: series 1901
Discovered By: Eugène Demarçay

Name: Fermium
Symbol: Fm
Atomic Number: 100
Atomic Weight: (257)
Group: Actinide
Date Discovered: series 1955
Discovered By: Albert Ghiorso and coworkers

Name: Fluorine
Symbol: F
Atomic Number: 9
Atomic Weight: 18.9984
Group: Halogens
Date Discovered: 1771
Discovered By: Karl Scheele (isolated by Henri Moissan 1886)

Name: Francium
Symbol: Fr
Atomic Number: 87
Atomic Weight: (223)
Group: Alkali metals
Date Discovered: 1939
Discovered By: Marguérite Perey

Name: Gadolinium
Symbol: Gd
Atomic Weight: 64
Atomic Number: 157.25
Group: Lanthanide
Date Discovered: series 1886
Discovered By: Paul Lecoq de Boisbaudran

Name: Gallium
Symbol: Ga
Atomic Number: 31
Atomic Weight: 69.723
Group: Other
Date Discovered: metals 1875
Discovered By: Paul Lecoq de Boisbaudran

Name: Germanium
Symbol: Ge
Atomic Number: 32
Atomic Weight: 72.61
Group: Other metals
Date Discovered: 1886
Discovered By: Clemens Winkler

Name: Gold
Symbol: Au
Atomic Number: 79
Atomic Weight: 196.9665
Group: Transition metals
Date Discovered: prehistoric
Discovered By: unknown

Name: Hafnium
Symbol: Hf
Atomic Number: 72
Atomic Weight: 178.49
Group: Transition metals
Date Discovered: 1913
Discovered By: Dirk Coster and Georg von Hevesy

Name: Hassium
Symbol: Hs
Atomic Number: 108
Atomic Weight: (263)
Group: Transition metals
Date Discovered: 1984
Discovered By: Peter Armbruster and coworkers

Name: Helium
Symbol: He
Atomic Number: 2
Atomic Weight: 4.0026
Group: Noble gases
Date Discovered: 1868
Discovered By: Pierre Janssen

Name: Holmium
Symbol: Ho
Atomic Number: 67
Atomic Weight: 164.9303
Group: Lanthanide
Date Discovered: series 1879
Discovered By: Per Cleve

Name: Hydrogen
Symbol: H
Atomic Number: 1
Atomic Weight: 1.0079
Group: Nonmetals
Date Discovered: 1766
Discovered By: Henry Cavendish

Alphabetic list of Chemical Elements (C-D)

.

Name: Cadmium
Symbol: Cd
Atomic Number: 48
Atomic Weight: 112.412
Group: Transition metals
Date Discovered: 1817
Discovered By: Friedrich Strohmeyer

Name: Calcium
Symbol: Ca
Atomic Number: 20
Atomic Weight: 40.078
Group: Alkaline earth metals
Date Discovered: 1808
Discovered By: Humphry Davy

Name: Californium
Symbol: Cf
Atomic Number: 98
Atomic Weight: (251)
Group: Actinide
Date Discovered: 1950
Discovered By: Glenn Seaborg, Stanley Thompson, Kenneth Street, Jr., and Albert Ghiorso

Name: Carbon
Symbol: C
Atomic Number: 6
Atomic Weight: 12.011
Group: Nonmetals
Date Discovered: prehistoric
Discovered By: unknown

Name: Cerium
Symbol: Ce
Atomic Number: 58
Atomic Weight: 140.115
Group: Lanthanide
Date Discovered: 1804
Discovered By: Jöns Berzelius and Wilhelm Hisinger, and independently by Martin Klaproth

Name: Cesium
Symbol: Cs
Atomic Number: 55
Atomic Weight: 132.9054
Group: Alkali metals
Date Discovered: 1860
Discovered By: Robert Bunsen and Gustav Kirchhoff

Name: Chlorine
Symbol: Cl
Atomic Number: 17
Atomic Weight: 35.4528
Group: Halogens
Date Discovered: 1774
Discovered By: Karl Scheele

Name: Chromium
Symbol: Cr
Atomic Number: 24
Atomic Weight: 51.9962
Group: Transition metals
Date Discovered: 1797
Discovered By: Louis-Nicolas Vauquelin

Name: Cobalt
Symbol: Co
Atomic Number: 27
Atomic Weight: 58.9332
Group: Transition metals
Date Discovered: 1730
Discovered By: Georg Brandt

Name: Copper
Symbol: Cu
Atomic Number: 29
Atomic Weight: 63.546
Group: Transition metals
Date Discovered: prehistoric
Discovered By: unknown

Name: Curium
Symbol: Cm
Atomic Number: 96
Atomic Weight: (247)
Group: Actinide
Date Discovered: 1944
Discovered By: Glenn Seaborg, Ralph James, and Albert Ghiorso

Name: Darmstadtium
Symbol: Ds
Atomic Number: 110
Atomic Weight: (271)
Group: Transition metals
Date Discovered: 1994
Discovered By: team at the Heavy-Ion Research Laboratory, Darmstadt, Germany

Name: Dubnium
Symbol: Db
Atomic Number: 105
Atomic Weight: (262)
Group: Transition metals
Date Discovered: 1970
Discovered By: claimed by Albert Ghiorso and coworkers (disputed by Soviet workers)

Name: Dysprosium
Symbol: Dy
Atomic Number: 66
Atomic Weight: 162.500
Group: Lanthanide
Date Discovered: series 1886
Discovered By: Paul Lecoq de Boisbaudran

Alphabetic list of Chemical Elements (A-B)

.

Name: Actinium
Symbol: Ac
Atomic Number: 89
Atomic Weight: (227)
Group: Actinide
Date Discovered: 1899
Discovered By: André Debierne

Name: Aluminum
Symbol: Al
Atomic Number: 13
Atomic Weight: 26.9815
Group: Other metals
Date Discovered: 1824
Discovered By: Hans Oersted (also attributed to Friedrich Wöhler 1827)

Name: Americium
Symbol: Am
Atomic Number: 95
Atomic Weight: 243
Group: Actinide
Date Discovered: 1944
Discovered By: Glenn Seaborg, Ralph James, Leon Morgan, and Albert Ghiorso

Name: Antimony
Symbol: Sb
Atomic Number: 51
Atomic Weight: 121.760
Group: Other metals
Date Discovered: prehistoric
Discovered By: unknown

Name: Argon
Symbol: Ar
Atomic Number: 18
Atomic Weight: 39.948
Group: Noble gases
Date Discovered: 1894
Discovered By: John Rayleigh and William Ramsay

Name: Arsenic
Symbol: As
Atomic Number: 33
Atomic Weight: 74.9216
Group: Nonmetals
Date Discovered: prehistoric
Discovered By: unknown

Name: Astatine
Symbol: At
Atomic Number: 85
Atomic Weight: (210)
Group: Halogens
Date Discovered: 1940
Discovered By: Dale R. Corson, K. R. MacKenzie, and Emilio Segrè

Name: Barium
Symbol: Ba
Atomic Number: 56
Atomic Weight: 137.328
Group: Alkaline earth metals
Date Discovered: 1808
Discovered By: Humphry Davy

Name: Berkelium
Symbol: Bk
Atomic Number: 97
Atomic Weight: (247)
Group: Actinide
Date Discovered: 1949
Discovered By: Glenn Seaborg, Stanley Thompson, and Albert Ghiorso

Name: Beryllium
Symbol: Be
Atomic Number: 4
Atomic Weight: 9.0122
Group: Alkaline earth metals
Date Discovered: 1798
Discovered By: Louis-Nicolas Vauquelin (isolated by Friedrich Wöhler and Antoine-Alexandre-Brutus Bussy 1828)

Name: Bismuth
Symbol: Bi
Atomic Number: 83
Atomic Weight: 208.9804
Group: Other metals
Date Discovered: prehistoric
Discovered By: unknown

Name: Bohrium
Symbol: Bh
Atomic Number: 107
Atomic Weight: (262)
Group: Transition metals
Date Discovered: 1976
Discovered By: Georgii Flerov and Yuri Oganessian (confirmed by German scientist Peter Armbruster and coworkers)

Name: Boron
Symbol: B
Atomic Number: 5
Atomic Weight: 10.81
Group: Nonmetals
Date Discovered: 1808
Discovered By: Humphry Davy, and independently by Joseph Gay-Lussac and Louis-Jacques Thénard

Name: Bromine
Symbol: Br
Atomic Number: 35
Atomic Weight: 79.904
Group: Halogens
Date Discovered: 1826
Discovered By: Antoine-Jérôme Balard

Labels:

Periodic Table

.
Periodic Table, table of the chemical elements arranged to illustrate patterns of recurring chemical and physical properties. Elements, such as oxygen, iron, and gold, are the most basic chemical substances and cannot be broken down by chemical reactions. All other substances are formed from combinations of elements. The periodic table provides a means of arranging all the known elements and even those yet to be discovered.

See Alphabetic list of Chemical Elements


ARRANGEMENT OF THE TABLE

The elements within the modern periodic table are arranged from left to right, top to bottom, in order of increasing atomic number. An element’s atomic number is the number of protons in its nucleus. There are 92 naturally occurring elements, ranging from hydrogen, which has atomic number 1, to uranium, whose atomic number is 92. The periodic table also includes artificially created elements, whose atomic numbers are higher than 92. These additions must be prepared through nuclear reactions. The most recent element to be synthesized was an element with 114 protons in the nucleus of each of its atoms. None of the artificially created elements with atomic numbers higher than 109 have yet been officially named.

Whereas the ordering of the elements is completely determined by their atomic numbers, the arrangement into vertical columns, called groups, is determined by a number of factors. These factors include chemical properties, physical properties, and the number of electrons thought to exist in the outer shells of the element’s atoms. (The electrons that surround the nucleus of an atom are arranged in concentric shells.)

The placement of elements into groups within the periodic table is not completely clear-cut. Some scientists disagree about minor differences in the placement of elements such as hydrogen and helium. Helium, which does not react with other elements, is usually placed in group 18, which houses the noble gases. This group also includes neon, argon, and krypton, all of which are also very unreactive. Scientists who group the elements based primarily on the number of outer-shell electrons place helium with elements such as magnesium, calcium, and barium among the alkaline earth metals of group 2. Elements in group 2 have two electrons in their outermost shell.

The periodic table has been published in various shapes and sizes, but the most commonly used modern form begins with a column of group 1 metals on the left-hand side, followed by a column of group 2 alkaline earth metals. These columns are followed by a block of 40 elements divided into ten columns of four elements each. The groups in this block, collectively called the transition metals, are numbered 3 through 12. Groups 13 through 18 make up the right-hand side of the table. A diagonal dividing line separates the nonmetals in the upper-right portion of this block, such as oxygen, carbon, and nitrogen, from the metals such as tin and lead in the lower left portion.

There is an additional block of 28 elements, divided into two rows of 14 elements each, that is usually placed beneath the main table. These are the rare earth elements, whose properties are all remarkably similar. They are so similar to one another that chemists have difficulty separating them when they occur together as mixtures. This additional block really belongs between the first block, consisting of groups 1 and 2, and the transition metal block. For convenience it is placed at the bottom of the table rather than in its proper place. Otherwise the periodic table would be very wide and would not lend itself to being represented on wall charts.

Scientists refer to the horizontal rows in the periodic table as periods. Periods vary in length. Moving through the table from top to bottom, the successive periods contain 2, 8, 8, 18, 18, 32, and 32 elements. These numbers correspond to the maximum number of electrons that can be accommodated in the largest electron shell in an atom of any element belonging to that period.

Labels:

Elements, Chemical

Elements, Chemical, substances that cannot be decomposed or broken into more elementary substances by ordinary chemical means. Elements were at one time believed to be the fundamental substances but are now known to consist of a number of different elementary particles. More than 100 chemical elements are known to exist in the universe, although several of these, the so-called transuranium elements, have not been found in nature, and can only be produced artificially. See also Chemistry.

Chemical elements are classified as metals and nonmetals. The atoms of metals are electropositive and combine readily with the electronegative atoms of the nonmetals. A group of elements called metalloids, intermediate in properties between the metals and the nonmetals, are sometimes considered a separate class. When the elements are arranged in the order of their atomic numbers (a number proportional to the net positive charge on the nucleus of an atom of an element), elements of similar physical and chemical properties occur at specific intervals (see Periodic Law). These groups of elements with similar physical and chemical properties are called families, examples of which are the alkaline earth metals, rare earth elements, halogens, and the noble gases.

The unit for atomic weight of the elements is one-twelfth of the weight of the carbon-12 atom, which is arbitrarily set at 12 (see Atom). The atomic number, weight, and chemical symbol of each of the known elements are given in the accompanying table. See articles on each element.

When two atoms have the same atomic number but different atomic weights, they are said to be isotopes. Many natural isotopes are known for some elements, whereas other elements occur in only one isotopic form. Hundreds of synthetic isotopes have been made. Some natural isotopes, and many synthetic ones, are unstable (see Isotope; Radioactivity).

The heavy transuranium elements are produced in particle accelerators by bombarding atomic nuclei with charged atomic nuclei or nuclear particles to form a heavier element. These superheavy elements are radioactive and decay into more stable, lighter elements rapidly. In 1996 physicists at the German National Laboratory for Heavy Ion Research in Darmstadt, Germany, created an element with 112 protons by bombarding lead with atoms of zinc. This element is named Ununbium (Uub). Some physicists speculate that a number of stable, superheavy elements may exist—elements with atomic numbers as high as 164. In 1998, scientists at Russia’s Joint Institute for Nuclear Research in Dubna announced they had created the first of these elements. After bombarding plutonium with atoms of calcium for weeks, they found evidence of one atom containing 114 protons that lasted for 30 seconds. Atoms of other heavy elements last for only a small fraction of a second.

related articles:

Chemistry overview
Inorganic Chemistry, study of noncarbon chemical compounds
Organic Chemistry, study of chemical compounds containing carbon
Periodic Law, predicting chemical properties
Periodic Table, arranging chemical elements in groups
alphabetic list of chemical elements
Labels:

Proton

Proton, elementary particle that carries a positive electric charge and, along with the electron and the neutron, is one of the building blocks of all atoms. Elementary particles are the smallest parts of matter that scientists can isolate. The proton is one of the few elementary particles that is stable—that is, it can exist by itself for a long period of time. Protons and neutrons are the building blocks of the atomic nucleus, the center of the atom. Electrons form the outer part of the atom. Protons have a positive electrical charge of 1.602 x 10-19 coulomb. This charge is equal but opposite to the negative charge of the electron. Neutrons have no electrical charge. Protons have a mass of 1.67 x 10-27 kg and, along with neutrons, they account for most of the mass in atoms. Atoms contain an equal number of protons and electrons so that every atom has an overall charge of zero.(See also Atom and Electricity)

The number of protons in the nucleus of an atom determines what kind of chemical element it is. All substances in nature are made up of combinations of the 92 different chemical elements, substances that cannot be broken into simpler substances by chemical processes. The atom is the smallest part of a chemical element that still retains the properties of the element. The number of protons in each atom can range from one in the hydrogen atom to 92 in the uranium atom, the heaviest naturally occurring element. (In the laboratory, scientists have created elements with as many as 116 protons in each nucleus.) The atomic number of an element is equal to the number of protons in each atom’s nucleus. The number of electrons in an uncharged atom must be equal to the number of protons, and the arrangement of these electrons determines the chemical properties of the atom.

STRUCTURE AND CHARACTERISTICS

The proton is 1,836 times as heavy as the electron. For an atom of hydrogen, which contains one electron and one proton, the proton provides 99.95 percent of the mass. The neutron weighs a little more than the proton. Elements heavier than hydrogen usually contain about the same number of protons and neutrons in their nuclei, so the atomic mass, or the mass of one atom, is usually about twice the atomic number.

Protons are affected by all four of the fundamental forces that govern all interactions between particles and energy in the universe. The electromagnetic force arises from matter carrying an electrical charge. It causes positively charged protons to attract negatively charged electrons and holds them in orbit around the nucleus of the atom. This force also makes the closely packed protons within the atomic nucleus repel each other with a force that is 100 million times stronger than the electrical attraction that binds the electrons. This repulsion is overcome, however, by the strong nuclear force, which binds the protons and neutrons together into a compact nucleus. The other two fundamental forces, gravitation and the weak nuclear force, also affect the proton. Gravitation is a force that attracts anything with mass (such as the proton) to every other thing in the universe that has mass. It is weak when the masses are small, but can become very large when the masses are great. The weak nuclear force is a feeble force that occurs between certain types of elementary particles, including the proton, and governs how some elementary particles break up into other particles.
Labels:

Neutron

Neutron, electrically neutral elementary particle that is part of the nucleus of the atom. Elementary particles are the smallest parts of matter that scientists can isolate. The neutron is about 10-13 cm in diameter and weighs 1.6749 x 10-27 kg. See also Atom.

Neutrons and protons bind tightly together to create atomic nuclei. The number of protons an atom contains determines which chemical element it is, ranging from 1 proton for hydrogen to 92 for uranium, the largest naturally occurring element. Each atom usually contains about as many neutrons as protons, but different atoms of the same element may have different numbers of neutrons.

Atoms that differ only in the number of neutrons are called isotopes. For example, most atoms of the simplest element, hydrogen, have a nucleus containing only a single proton. In natural hydrogen, however, 0.015 percent of the atoms have a neutron in addition to the proton. This isotope is called heavy hydrogen or deuterium. An element usually has several isotopes, all nearly identical in the way they react chemically with other elements and each other. Scientists can distinguish different isotopes of an element by examining properties of the element’s nuclei, such as the mass of the nucleus.

CHARACTERISTICS

The neutron is slightly heavier than a proton and 1,838 times as heavy as the electron. It is affected by all the four fundamental forces of nature. Because it has mass, it is affected by gravitation, the force of attraction between all objects in the universe. Although the neutron has no electrical charge, it is slightly magnetic, so it is affected by the electromagnetic force, the force of attraction or repulsion between electrically charged or magnetic objects. The neutron is affected by the strong nuclear force, an attraction that binds the neutron to protons and other neutrons in the nucleus. The neutron is also affected by the weak nuclear force, an interaction among the building blocks of the neutron that causes the neutron to decay, or break apart. Isolated from nuclear matter, a free neutron decays into a positively charged proton and a negatively charged electron, releasing energy in the process (see Nuclear Energy). The average lifetime of a free neutron is just under 15 minutes.
Labels:

Electron

Electron, negatively charged particle found in an atom. Electrons, along with neutrons and protons, comprise the basic building blocks of all atoms. The electrons form the outer layer or layers of an atom, while the neutrons and protons make up the nucleus, or core, of the atom. Electrons, neutrons, and protons are elementary particles—that is, they are among the smallest parts of matter that scientists can isolate. The electron carries a negative electric charge of –1.602 x 10-19 coulomb and has a mass of 9.109 x 10-31 kg. See also Atom.

Electrons are responsible for many important physical phenomena, such as electricity and light, and for physical and chemical properties of matter. Electrons form electric currents by flowing in a stream and carrying their negative charge with them. All electrical devices, from flashlights to computers, depend on the movement of electrons. Electrons also are involved in creating light. The electrons in the outer layers of the atom sometimes lose energy, emitting the energy in the form of light. Because electrons form the outer layers of atoms, they are also responsible for many of the physical and chemical properties of the chemical elements. Electrons help determine how atoms of an element behave with respect to each other and how they react with atoms of other elements. See also Chemistry.
Labels:

Atom

Atom, tiny basic building block of matter. All the material on Earth is composed of various combinations of atoms. Atoms are the smallest particles of a chemical element that still exhibit all the chemical properties unique to that element. A row of 100 million atoms would be only about a centimeter long. See also Chemical Element.

Understanding atoms is key to understanding the physical world. More than 100 different elements exist in nature, each with its own unique atomic makeup. The atoms of these elements react with one another and combine in different ways to form a virtually unlimited number of chemical compounds. When two or more atoms combine, they form a molecule. For example, two atoms of the element hydrogen (abbreviated H) combine with one atom of the element oxygen (O) to form a molecule of water (H20).

Since all matter—from its formation in the early universe to present-day biological systems—consists of atoms, understanding their structure and properties plays a vital role in physics, chemistry, and medicine. In fact, knowledge of atoms is essential to the modern scientific understanding of the complex systems that govern the physical and biological worlds. Atoms and the compounds they form play a part in almost all processes that occur on Earth and in space. All organisms rely on a set of chemical compounds and chemical reactions to digest food, transport energy, and reproduce. Stars such as the Sun rely on reactions in atomic nuclei to produce energy. Scientists duplicate these reactions in laboratories on Earth and study them to learn about processes that occur throughout the universe.

THE STRUCTURE OF THE ATOM

Atoms are made of smaller particles, called electrons, protons, and neutrons. An atom consists of a cloud of electrons surrounding a small, dense nucleus of protons and neutrons. Electrons and protons have a property called electric charge, which affects the way they interact with each other and with other electrically charged particles. Electrons carry a negative electric charge, while protons have a positive electric charge. The negative charge is the opposite of the positive charge, and, like the opposite poles of a magnet, these opposite electric charges attract one another. Conversely, like charges (negative and negative, or positive and positive) repel one another. The attraction between an atom’s electrons and its protons holds the atom together. Normally, an atom is electrically neutral, which means that the negative charge of its electrons is exactly equaled by the positive charge of its protons.

The nucleus contains nearly all of the mass of the atom, but it occupies only a tiny fraction of the space inside the atom. The diameter of a typical nucleus is only about 1 × 10-14 m (4 × 10-13 in), or about 1/100,000 of the diameter of the entire atom. The electron cloud makes up the rest of the atom’s overall size. If an atom were magnified until it was as large as a football stadium, the nucleus would be about the size of a grape.
Labels:

Elementary Particles

.
Elementary Particles, in physics, particles that cannot be broken down into any other particles. The term elementary particles also is used more loosely to include some subatomic particles that are composed of other particles. Particles that cannot be broken further are sometimes called fundamental particles to avoid confusion. These fundamental particles provide the basic units that make up all matter and energy in the universe.

Scientists and philosophers have sought to identify and study elementary particles since ancient times. Aristotle and other ancient Greek philosophers believed that all things were composed of four elementary materials: fire, water, air, and earth. People in other ancient cultures developed similar notions of basic substances. As early scientists began collecting and analyzing information about the world, they showed that these materials were not fundamental but were made of other substances.

In the 1800s British physicist John Dalton was so sure he had identified the most basic objects that he called them atoms (from the Greek word for “indivisible”). By the early 1900s scientists were able to break apart these atoms into particles that they called the electron and the nucleus. Electrons surround the dense nucleus of an atom. In the 1930s, researchers showed that the nucleus consists of smaller particles, called the proton and the neutron. Today, scientists have evidence that the proton and neutron are themselves made up of even smaller particles, called quarks.

Topics:
Labels:

Force

Force, in physics, any action or influence that accelerates an object. Force is a vector, which means that it has both direction and magnitude. When several forces act on an object, the forces can be combined to give a net force. The net force acting on an object, the object's mass, and the acceleration of the object are all related to each other by Newton's second law of motion, named after English physicist and mathematician Isaac Newton. The second law of motion states that the acceleration an object experiences multiplied by the mass of the object is equal to the net force acting on an object. Thus, if a given force acts on two objects of different mass, the object with a larger mass will have a lower acceleration. See Mechanics.

An object experiences a force when it is pushed or pulled by another object. For example, shoving a stationary shopping cart applies a force that causes the shopping cart to accelerate. An object can also experience a force because of the influence of a field. For example, a dropped ball accelerates toward the ground because of the presence of the gravitational field (see Gravitation); electrical charges attract or repel each other because of the presence of an electric field (see Electricity).

Usually, several forces act on an object at once. If multiple forces combine to give a net force that is zero, then the object will not accelerate; the object will either remain motionless or continue moving at a constant velocity. For example, if a person pushes a shopping cart with a force equal in magnitude to the force of friction that opposes the cart's motion, the forces will cancel, giving a net force of zero. As a result, the cart will move down the aisle with a constant velocity. If the person suddenly stops pushing the cart, the only force acting on the cart is the frictional force. Since the net force is no longer zero, the cart accelerates: its velocity drops to zero.

In the international system of units, the unit of force is the newton, which is the force that imparts to an object with a mass of 1 kg an acceleration of 1 m/sec2. In English units, the unit of force is the poundal, which is the amount of force that accelerates a 1-lb object 1 ft/sec2.

Forces acting at the molecular and atomic level are also known as interactions. See also Elementary Particles.
Labels:

Gravitation

.
Gravitation, the force of attraction between all objects that tends to pull them toward one another. It is a universal force, affecting the largest and smallest objects, all forms of matter, and energy. Gravitation governs the motion of astronomical bodies. It keeps the moon in orbit around the earth and keeps the earth and the other planets of the solar system in orbit around the sun. On a larger scale, it governs the motion of stars and slows the outward expansion of the entire universe because of the inward attraction of galaxies to other galaxies. Typically the term gravitation refers to the force in general, and the term gravity refers to the earth's gravitational pull.

Gravitation is one of the four fundamental forces of nature, along with electromagnetism and the weak and strong nuclear forces, which hold together the particles that make up atoms. Gravitation is by far the weakest of these forces and, as a result, is not important in the interactions of atoms and nuclear particles or even of moderate-sized objects, such as people or cars. Gravitation is important only when very large objects, such as planets, are involved. This is true for several reasons. First, the force of gravitation reaches great distances, while nuclear forces operate only over extremely short distances and decrease in strength very rapidly as distance increases. Second, gravitation is always attractive. In contrast, electromagnetic forces between particles can be repulsive or attractive depending on whether the particles both have a positive or negative electrical charge, or they have opposite electrical charges (see Electricity). These attractive and repulsive forces tend to cancel each other out, leaving only a weak net force. Gravitation has no repulsive force and, therefore, no such cancellation or weakening.

The gravitational attraction of objects for one another is the easiest fundamental force to observe and was the first fundamental force to be described with a complete mathematical theory by the English physicist and mathematician Sir Isaac Newton. A more accurate theory called general relativity was formulated early in the 20th century by the German-born American physicist Albert Einstein. Scientists recognize that even this theory is not correct for describing how gravitation works in certain circumstances, and they continue to search for an improved theory.

EARTH'S GRAVITATION

Gravitation plays a crucial role in most processes on the earth. The ocean tides are caused by the gravitational attraction of the moon and the sun on the earth and its oceans. Gravitation drives weather patterns by making cold air sink and displace less dense warm air, forcing the warm air to rise. The gravitational pull of the earth on all objects holds the objects to the surface of the earth. Without it, the spin of the earth would send them floating off into space.

The gravitational attraction of every bit of matter in the earth for every other bit of matter amounts to an inward pull that holds the earth together against the pressure forces tending to push it outward. Similarly, the inward pull of gravitation holds stars together. When a star's fuel nears depletion, the processes producing the outward pressure weaken and the inward pull of gravitation eventually compresses the star to a very compact size (see Star, Black Hole).

Acceleration

If an object held near the surface of the earth is released, it will fall and accelerate, or pick up speed, as it descends. This acceleration is caused by gravity, the force of attraction between the object and the earth. The force of gravity on an object is also called the object's weight. This force depends on the object's mass, or the amount of matter in the object. The weight of an object is equal to the mass of the object multiplied by the acceleration due to gravity.

A bowling ball that weighs 16 lb is actually being pulled toward the earth with a force of 16 lb. In the metric system, the bowling ball is pulled toward the earth with a force of 71 newtons (a metric unit of force abbreviated N). The bowling ball also pulls on the earth with a force of 16 lb (71 N), but the earth is so massive that it does not move appreciably. In order to hold the bowling ball up and keep it from falling, a person must exert an upward force of 16 lb (71 N) on the ball. This upward force acts to oppose the 16 lb (71 N) downward weight force, leaving a total force of zero. The total force on an object determines the object's acceleration.

If the pull of gravity is the only force acting on an object, then all objects, regardless of their weight, size, or shape, will accelerate in the same manner. At the same place on the earth, the 16 lb (71 N) bowling ball and a 500 lb (2200 N) boulder will fall with the same rate of acceleration. As each second passes, each object will increase its downward speed by about 9.8 m/sec (32 ft/sec), resulting in an acceleration of 9.8 m/sec/sec (32 ft/sec/sec). In principle, a rock and a feather both would fall with this acceleration if there were no other forces acting. In practice, however, air friction exerts a greater upward force on the falling feather than on the rock and makes the feather fall more slowly than the rock.

The mass of an object does not change as it is moved from place to place, but the acceleration due to gravity, and therefore the object's weight, will change because the strength of the earth's gravitational pull is not the same everywhere. The earth's pull and the acceleration due to gravity decrease as an object moves farther away from the center of the earth. At an altitude of 4000 miles (6400 km) above the earth's surface, for instance, the bowling ball that weighed 16 lb (71 N) at the surface would weigh only about 4 lb (18 N). Because of the reduced weight force, the rate of acceleration of the bowling ball at that altitude would be only one quarter of the acceleration rate at the surface of the earth. The pull of gravity on an object also changes slightly with latitude. Because the earth is not perfectly spherical, but bulges at the equator, the pull of gravity is about 0.5 percent stronger at the earth's poles than at the equator.
Labels:

Quantum Theory

Quantum Theory, in physics, description of the particles that make up matter and how they interact with each other and with energy. Quantum theory explains in principle how to calculate what will happen in any experiment involving physical or biological systems, and how to understand how our world works. The name “quantum theory” comes from the fact that the theory describes the matter and energy in the universe in terms of single indivisible units called quanta (singular quantum). Quantum theory is different from classical physics. Classical physics is an approximation of the set of rules and equations in quantum theory. Classical physics accurately describes the behavior of matter and energy in the everyday universe. For example, classical physics explains the motion of a car accelerating or of a ball flying through the air. Quantum theory, on the other hand, can accurately describe the behavior of the universe on a much smaller scale, that of atoms and smaller particles. The rules of classical physics do not explain the behavior of matter and energy on this small scale. Quantum theory is more general than classical physics, and in principle, it could be used to predict the behavior of any physical, chemical, or biological system. However, explaining the behavior of the everyday world with quantum theory is too complicated to be practical.

Quantum theory not only specifies new rules for describing the universe but also introduces new ways of thinking about matter and energy. The tiny particles that quantum theory describes do not have defined locations, speeds, and paths like objects described by classical physics. Instead, quantum theory describes positions and other properties of particles in terms of the chances that the property will have a certain value. For example, it allows scientists to calculate how likely it is that a particle will be in a certain position at a certain time.

Quantum description of particles allows scientists to understand how particles combine to form atoms. Quantum description of atoms helps scientists understand the chemical and physical properties of molecules, atoms, and subatomic particles. Quantum theory enabled scientists to understand the conditions of the early universe, how the Sun shines, and how atoms and molecules determine the characteristics of the material that they make up. Without quantum theory, scientists could not have developed nuclear energy or the electric circuits that provide the basis for computers.

Quantum theory describes all of the fundamental forces—except gravitation—that physicists have found in nature. The forces that quantum theory describes are the electrical, the magnetic, the weak, and the strong. Physicists often refer to these forces as interactions, because the forces control the way particles interact with each other. Interactions also affect spontaneous changes in isolated particles.
Labels:

Mass

Mass (physics), in physics, amount of matter that a body contains, and a measure of the inertial property of that body, that is, of its resistance to change of motion (see Inertia). Mass is different from weight, which is a measure of the attraction of the earth for a given mass (see Gravitation). Inertial mass and gravitational mass are identical. Weight, although proportional to mass, varies with the position of a given mass relative to the earth; thus, equal masses at the same location in a gravitational field will have equal weights. A mass in interstellar space may have nearly zero weight. A fundamental principle of classical physics is the law of conservation of mass, which states that matter cannot be created or destroyed. This law holds true in chemical reactions but is modified in cases where atoms disintegrate and matter is converted to energy or energy is converted to matter (see Nuclear Energy; X Ray).

The theory of relativity, initially formulated in 1905 by the German-born American physicist Albert Einstein, did much to change traditional concepts of mass. In modern physics, the mass of an object is regarded as changing as its velocity approaches that of light, that is, when it approaches 300,000 km/sec (about 186,000 mi/sec); an object moving at a speed of approximately 260,000 km/sec (about 160,000 mi/sec), for example, has a mass about double its so-called rest mass. Where such velocities are involved, as in nuclear reactions, mass can be converted into energy and vice versa, as suggested by Einstein in his famous equation E = mc2 (energy equals mass multiplied by the velocity of light squared).

See also International System of Units; Mechanics; Quantum Theory.
Labels:
Subscribe to: Posts (Atom)

Popular Posts

  • Electricity and Magnetism
    . Many similarities exist between electric and magnetic phenomena. A magnet has two opposite poles, referred to as north and south. Opposit...
  • Ohm’s Law
    . The relationship between current, voltage, and resistance is given by Ohm’s law. This law states that the amount of current passing throu...
  • Quantum Theory
    Quantum Theory, in physics, description of the particles that make up matter and how they interact with each other and with energy. Quantum ...
  • Periodic Table
    . Periodic Table, table of the chemical elements arranged to illustrate patterns of recurring chemical and physical properties. Elements, su...
  • Electromagnetic Radiation
    Electromagnetic Radiation is the energy waves produced by the oscillation or acceleration of an electric charge. Electromagnetic waves have ...