Tuesday, May 3, 2011

Periodic Table




The periodic table of the chemical elements (also periodic table of the elements or just the periodic table) is a tabular display of the chemical elements. Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869, who intended the table to illustrate recurring ("periodic") trends in the properties of the elements. The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior.[1]

The periodic table is now ubiquitous within the academic discipline of chemistry, providing a useful framework to classify, systematize, and compare all of the many different forms of chemical behavior. The table has found many applications in chemistry, physics, biology, and engineering, especially chemical engineering. The current standard table contains 118 elements to date. (elements 1–118).

This common arrangement of the periodic table separates the lanthanoids and actinoids (the f-block) from other elements. The wide periodic table incorporates the f-block. The extended periodic table adds the 8th and 9th periods, incorporating the f-block and adding the theoretical g-block.



Elements of the Periodic Table

Elements Sorted by Name
Name Symbol Atomic Number Atomic Weight Group Date Discovered Discovered By
Actinium Ac 89 (227) Actinide series 1899 André Debierne
Aluminum Al 13 26.9815 Other metals 1824 Hans Oersted (also attributed to Friedrich Wöhler 1827)
Americium Am 95 243 Actinide series 1944 Glenn Seaborg, Ralph James, Leon Morgan, and Albert Ghiorso
Antimony Sb 51 121.760 Other metals prehistoric unknown
Argon Ar 18 39.948 Noble gases 1894 John Rayleigh and William Ramsay
Arsenic As 33 74.9216 Nonmetals prehistoric unknown
Astatine At 85 (210) Halogens 1940 Dale R. Corson, K. R. MacKenzie, and Emilio Segrè
Barium Ba 56 137.328 Alkaline earth metals 1808 Humphry Davy
Berkelium Bk 97 (247) Actinide series 1949 Glenn Seaborg, Stanley Thompson, and Albert Ghiorso
Beryllium Be 4 9.0122 Alkaline earth metals 1798 Louis-Nicolas Vauquelin (isolated by Friedrich Wöhler and Antoine-Alexandre-Brutus Bussy 1828)
Bismuth Bi 83 208.9804 Other metals prehistoric unknown
Bohrium Bh 107 (262) Transition metals 1976 Georgii Flerov and Yuri Oganessian (confirmed by German scientist Peter Armbruster and coworkers)
Boron B 5 10.81 Nonmetals 1808 Humphry Davy, and independently by Joseph Gay-Lussac and Louis-Jacques Thénard
Bromine Br 35 79.904 Halogens 1826 Antoine-Jérôme Balard
Cadmium Cd 48 112.412 Transition metals 1817 Friedrich Strohmeyer
Calcium Ca 20 40.078 Alkaline earth metals 1808 Humphry Davy
Californium Cf 98 (251) Actinide series 1950 Glenn Seaborg, Stanley Thompson, Kenneth Street, Jr., and Albert Ghiorso
Carbon C 6 12.011 Nonmetals prehistoric unknown
Cerium Ce 58 140.115 Lanthanide series 1804 Jöns Berzelius and Wilhelm Hisinger, and independently by Martin Klaproth
Cesium Cs 55 132.9054 Alkali metals 1860 Robert Bunsen and Gustav Kirchhoff
Chlorine Cl 17 35.4528 Halogens 1774 Karl Scheele
Chromium Cr 24 51.9962 Transition metals 1797 Louis-Nicolas Vauquelin
Cobalt Co 27 58.9332 Transition metals 1730 Georg Brandt
Copper Cu 29 63.546 Transition metals prehistoric unknown
Curium Cm 96 (247) Actinide series 1944 Glenn Seaborg, Ralph James, and Albert Ghiorso
Darmstadtium Ds 110 (271) Transition metals 1994 team at the Heavy-Ion Research Laboratory, Darmstadt, Germany
Dubnium Db 105 (262) Transition metals 1970 claimed by Albert Ghiorso and coworkers (disputed by Soviet workers)
Dysprosium Dy 66 162.500 Lanthanide series 1886 Paul Lecoq de Boisbaudran
Einsteinium Es 99 (252) Actinide series 1952 Albert Ghiorso and coworkers
Erbium Er 68 167.26 Lanthanide series 1843 Carl Mosander
Europium Eu 63 151.966 Lanthanide series 1901 Eugène Demarçay
Fermium Fm 100 (257) Actinide series 1955 Albert Ghiorso and coworkers
Fluorine F 9 18.9984 Halogens 1771 Karl Scheele (isolated by Henri Moissan 1886)
Francium Fr 87 (223) Alkali metals 1939 Marguérite Perey
Gadolinium Gd 64 157.25 Lanthanide series 1886 Paul Lecoq de Boisbaudran
Gallium Ga 31 69.723 Other metals 1875 Paul Lecoq de Boisbaudran
Germanium Ge 32 72.61 Other metals 1886 Clemens Winkler
Gold Au 79 196.9665 Transition metals prehistoric unknown
Hafnium Hf 72 178.49 Transition metals 1913 Dirk Coster and Georg von Hevesy
Hassium Hs 108 (263) Transition metals 1984 Peter Armbruster and coworkers
Helium He 2 4.0026 Noble gases 1868 Pierre Janssen
Holmium Ho 67 164.9303 Lanthanide series 1879 Per Cleve
Hydrogen H 1 1.0079 Nonmetals 1766 Henry Cavendish
Indium In 49 114.818 Other metals 1863 Ferdinand Reich and Hieronymus Richter
Iodine I 53 126.9045 Halogens 1811 Bernard Courtois
Iridium Ir 77 192.217 Transition metals 1804 Smithson Tennant
Iron Fe 26 55.845 Transition metals prehistoric unknown
Krypton Kr 36 83.798 Noble gases 1898 William Ramsay and Morris Travers
Lanthanum La 57 138.9055 Lanthanide series 1839 Carl Mosander
Lawrencium Lr 103 (260) Transition metals 1961 Albert Ghiorso, Torbjørn Sikkeland, Almon Larsh, and Robert Latimer
Lead Pb 82 207.2 Other metals prehistoric unknown
Lithium Li 3 6.941 Alkali metals 1817 Johan Arfwedson
Lutetium Lu 71 174.967 Transition metals 1907 Georges Urbain and Carl von Welsbach, independently of each other
Magnesium Mg 12 24.3051 Alkaline earth metals 1755 Joseph Black (oxide isolated by Humphry Davy 1808; pure form isolated by Antoine-Alexandre-Brutus Bussy 1828)
Manganese Mn 25 54.938 Transition metals 1774 Johann Gottlieb Gahn
Meitnerium Mt 109 (268) Transition metals 1982 Peter Armbruster and coworkers
Mendelevium Md 101 (258) Actinide series 1955 Albert Ghiorso, Bernard G. Harvey, Gregory Choppin, Stanley Thompson, and Glenn Seaborg
Mercury Hg 80 200.59 Transition metals prehistoric unknown
Molybdenum Mo 42 95.94 Transition metals 1781 named by Karl Scheele (isolated by Peter Jacob Hjelm 1782)
Neodymium Nd 60 144.24 Lanthanide series 1885 Carl von Welsbach
Neon Ne 10 20.1798 Noble gases 1898 William Ramsay and Morris Travers
Neptunium Np 93 (237) Actinide series 1940 Edwin McMillan and Philip Abelson
Nickel Ni 28 58.6934 Transition metals 1751 Axel Cronstedt
Niobium Nb 41 92.9064 Transition metals 1801 Charles Hatchett
Nitrogen N 7 14.0067 Nonmetals 1772 Daniel Rutherford
Nobelium No 102 (259) Actinide series 1958 Albert Ghiorso, Torbjørn Sikkeland, J. R. Walton, and Glenn Seaborg
Osmium Os 76 190.23 Transition metals 1804 Smithson Tennant
Oxygen O 8 15.9994 Nonmetals 1774 Joseph Priestley and Karl Scheele, independently of each other
Palladium Pd 46 106.42 Transition metals 1804 William Wollaston
Phosphorus P 15 30.9738 Nonmetals 1674 Hennig Brand
Platinum Pt 78 195.08 Transition metals 1557 Julius Scaliger
Plutonium Pu 94 (244) Actinide series 1940 Glenn Seaborg, Edwin McMillan, Joseph Kennedy, and Arthur Wahl
Polonium Po 84 (209) Other metals 1898 Marie and Pierre Curie
Potassium K 19 39.0983 Alkali metals 1807 Humphry Davy
Praseodymium Pr 59 140.908 Lanthanide series 1885 Carl von Welsbach
Promethium Pm 61 (145) Lanthanide series 1945 J. A. Marinsky, Lawrence Glendenin, and Charles Coryell
Protactinium Pa 91 231.036 Actinide series 1913 Kasimir Fajans and O. Göhring
Radium Ra 88 (226) Alkaline earth metals 1898 Marie Curie
Radon Rn 86 (222) Noble gases 1900 Friedrich Dorn
Rhenium Re 75 186.207 Transition metals 1925 Walter Noddack, Ida Tacke, and Otto Berg
Rhodium Rh 45 102.9055 Transition metals 1804 William Wollaston
Rubidium Rb 37 85.4678 Alkali metals 1861 Robert Bunsen and Gustav Kirchhoff
Ruthenium Ru 44 101.07 Transition metals 1827 G. W. Osann (isolated by Karl Klaus 1844)
Rutherfordium Rf 104 (261) Transition metals 1969 claimed by U.S. scientist Albert Ghiorso and coworkers (disputed by Soviet workers)
Samarium Sm 62 150.36 Lanthanide series 1879 Paul Lecoq de Boisbaudran
Scandium Sc 21 44.9559 Transition metals 1876 Lars Nilson
Seaborgium Sg 106 (266) Transition metals 1974 claimed by Georgii Flerov and coworkers, and independently by Albert Ghiorso and coworkers
Selenium Se 34 78.96 Nonmetals 1817 Jöns Berzelius
Silicon Si 14 28.0855 Nonmetals 1823 Johan Arfwedson
Silver Ag 47 107.8682 Transition metals prehistoric unknown
Sodium Na 11 22.9898 Alkali metals 1807 Humphry Davy
Strontium Sr 38 87.62 Alkaline earth metals 1808 Humphry Davy
Sulfur S 16 32.067 Nonmetals prehistoric unknown
Tantalum Ta 73 180.948 Transition metals 1802 Anders Ekeberg
Technetium Tc 43 (98) Transition metals 1937 Carlo Perrier and Emilio Segrè
Tellurium Te 52 127.60 Nonmetals 1782 Franz Müller
Terbium Tb 65 158.9253 Lanthanide series 1843 Carl Mosander
Thallium Tl 81 204.3833 Other metals 1861 William Crookes (isolated by William Crookes and Claude August Lamy, independently of each other, in 1862)
Thorium Th 90 232.0381 Actinide series 1828 Jöns Berzelius
Thulium Tm 69 168.9342 Lanthanide series 1879 Per Cleve
Tin Sn 50 118.711 Other metals prehistoric unknown
Titanium Ti 22 47.867 Transition metals 1790 William Gregor
Tungsten W 74 183.84 Transition metals 1783 isolated by Juan José Elhuyar and Fausto Elhuyar
Ununbium Uub 112 (277) Transition metals 1996 team at the Heavy-Ion Research Laboratory, Darmstadt, Germany
Ununhexium Uuh 116 (292) Other metals 2000 team at the Joint Institute for Nuclear Research, Dubna, Russia
Ununquadium Uuq 114 (285) Other metals 1998 team at the Joint Institute for Nuclear Research, Dubna, Russia
Roentgenium Rg 111 (272) Transition metals 1994 team at the Heavy-Ion Research Laboratory, Darmstadt, Germany
Uranium U 92 238.0289 Actinide series 1789 Martin Klaproth (isolated by Eugène Péligot 1841)
Vanadium V 23 50.9415 Transition metals 1801 Andrés del Rio (disputed), or Nils Sefström 1830
Xenon Xe 54 131.29 Noble gases 1898 William Ramsay and Morris Travers
Ytterbium Yb 70 173.04 Lanthanide series 1878 Jean Charles de Marignac
Yttrium Y 39 88.906 Transition metals 1794 Johan Gadolin
Zinc Zn 30 65.409 Transition metals prehistoric unknown
Zirconium Zr 40 91.224 Transition metals 1789 Martin Klaproth
Elements Sorted by Atomic Number
Atomic Number Name Symbol Atomic Weight Group Date Discovered Discovered By
1 Hydrogen H 1.0079 Nonmetals 1766 Henry Cavendish
2 Helium He 4.0026 Noble gases 1868 Pierre Janssen
3 Lithium Li 6.941 Alkali metals 1817 Johan Arfwedson
4 Beryllium Be 9.0122 Alkaline earth metals 1798 Louis-Nicolas Vauquelin (isolated by Friedrich Wöhler and Antoine-Alexandre-Brutus Bussy 1828)
5 Boron B 10.81 Nonmetals 1808 Humphry Davy, and independently by Joseph Gay-Lussac and Louis-Jacques Thénard
6 Carbon C 12.011 Nonmetals prehistoric unknown
7 Nitrogen N 14.0067 Nonmetals 1772 Daniel Rutherford
8 Oxygen O 15.9994 Nonmetals 1774 Joseph Priestley and Karl Scheele, independently of each other
9 Fluorine F 18.9984 Halogens 1771 Karl Scheele (isolated by Henri Moissan 1886)
10 Neon Ne 20.1798 Noble gases 1898 William Ramsay and Morris Travers
11 Sodium Na 22.9898 Alkali metals 1807 Humphry Davy
12 Magnesium Mg 24.3051 Alkaline earth metals 1755 Joseph Black (oxide isolated by Humphry Davy 1808; pure form isolated by Antoine-Alexandre-Brutus Bussy 1828)
13 Aluminum Al 26.9815 Other metals 1824 Hans Oersted (also attributed to Friedrich Wöhler 1827)
14 Silicon Si 28.0855 Nonmetals 1823 Johan Arfwedson
15 Phosphorus P 30.9738 Nonmetals 1674 Hennig Brand
16 Sulfur S 32.067 Nonmetals prehistoric unknown
17 Chlorine Cl 35.4528 Halogens 1774 Karl Scheele
18 Argon Ar 39.948 Noble gases 1894 John Rayleigh and William Ramsay
19 Potassium K 39.0983 Alkali metals 1807 Humphry Davy
20 Calcium Ca 40.078 Alkaline earth metals 1808 Humphry Davy
21 Scandium Sc 44.9559 Transition metals 1876 Lars Nilson
22 Titanium Ti 47.867 Transition metals 1790 William Gregor
23 Vanadium V 50.9415 Transition metals 1801 Andrés del Rio (disputed), or Nils Sefström 1830
24 Chromium Cr 51.9962 Transition metals 1797 Louis-Nicolas Vauquelin
25 Manganese Mn 54.938 Transition metals 1774 Johann Gottlieb Gahn
26 Iron Fe 55.845 Transition metals prehistoric unknown
27 Cobalt Co 58.9332 Transition metals 1730 Georg Brandt
28 Nickel Ni 58.6934 Transition metals 1751 Axel Cronstedt
29 Copper Cu 63.546 Transition metals prehistoric unknown
30 Zinc Zn 65.409 Transition metals prehistoric unknown
31 Gallium Ga 69.723 Other metals 1875 Paul Lecoq de Boisbaudran
32 Germanium Ge 72.61 Other metals 1886 Clemens Winkler
33 Arsenic As 74.9216 Nonmetals prehistoric unknown
34 Selenium Se 78.96 Nonmetals 1817 Jöns Berzelius
35 Bromine Br 79.904 Halogens 1826 Antoine-Jérôme Balard
36 Krypton Kr 83.798 Noble gases 1898 William Ramsay and Morris Travers
37 Rubidium Rb 85.4678 Alkali metals 1861 Robert Bunsen and Gustav Kirchhoff
38 Strontium Sr 87.62 Alkaline earth metals 1808 Humphry Davy
39 Yttrium Y 88.906 Transition metals 1794 Johan Gadolin
40 Zirconium Zr 91.224 Transition metals 1789 Martin Klaproth
41 Niobium Nb 92.9064 Transition metals 1801 Charles Hatchett
42 Molybdenum Mo 95.94 Transition metals 1781 named by Karl Scheele (isolated by Peter Jacob Hjelm 1782)
43 Technetium Tc (98) Transition metals 1937 Carlo Perrier and Emilio Segrè
44 Ruthenium Ru 101.07 Transition metals 1827 G. W. Osann (isolated by Karl Klaus 1844)
45 Rhodium Rh 102.9055 Transition metals 1804 William Wollaston
46 Palladium Pd 106.42 Transition metals 1804 William Wollaston
47 Silver Ag 107.8682 Transition metals prehistoric unknown
48 Cadmium Cd 112.412 Transition metals 1817 Friedrich Strohmeyer
49 Indium In 114.818 Other metals 1863 Ferdinand Reich and Hieronymus Richter
50 Tin Sn 118.711 Other metals prehistoric unknown
51 Antimony Sb 121.760 Other metals prehistoric unknown
52 Tellurium Te 127.60 Nonmetals 1782 Franz Müller
53 Iodine I 126.9045 Halogens 1811 Bernard Courtois
54 Xenon Xe 131.29 Noble gases 1898 William Ramsay and Morris Travers
55 Cesium Cs 132.9054 Alkali metals 1860 Robert Bunsen and Gustav Kirchhoff
56 Barium Ba 137.328 Alkaline earth metals 1808 Humphry Davy
57 Lanthanum La 138.9055 Lanthanide series 1839 Carl Mosander
58 Cerium Ce 140.115 Lanthanide series 1804 Jöns Berzelius and Wilhelm Hisinger, and independently by Martin Klaproth
59 Praseodymium Pr 140.908 Lanthanide series 1885 Carl von Welsbach
60 Neodymium Nd 144.24 Lanthanide series 1885 Carl von Welsbach
61 Promethium Pm (145) Lanthanide series 1945 J. A. Marinsky, Lawrence Glendenin, and Charles Coryell
62 Samarium Sm 150.36 Lanthanide series 1879 Paul Lecoq de Boisbaudran
63 Europium Eu 151.966 Lanthanide series 1901 Eugène Demarçay
64 Gadolinium Gd 157.25 Lanthanide series 1886 Paul Lecoq de Boisbaudran
65 Terbium Tb 158.9253 Lanthanide series 1843 Carl Mosander
66 Dysprosium Dy 162.500 Lanthanide series 1886 Paul Lecoq de Boisbaudran
67 Holmium Ho 164.9303 Lanthanide series 1879 Per Cleve
68 Erbium Er 167.26 Lanthanide series 1843 Carl Mosander
69 Thulium Tm 168.9342 Lanthanide series 1879 Per Cleve
70 Ytterbium Yb 173.04 Lanthanide series 1878 Jean Charles de Marignac
71 Lutetium Lu 174.967 Transition metals 1907 Georges Urbain and Carl von Welsbach, independently of each other
72 Hafnium Hf 178.49 Transition metals 1913 Dirk Coster and Georg von Hevesy
73 Tantalum Ta 180.948 Transition metals 1802 Anders Ekeberg
74 Tungsten W 183.84 Transition metals 1783 isolated by Juan José Elhuyar and Fausto Elhuyar
75 Rhenium Re 186.207 Transition metals 1925 Walter Noddack, Ida Tacke, and Otto Berg
76 Osmium Os 190.23 Transition metals 1804 Smithson Tennant
77 Iridium Ir 192.217 Transition metals 1804 Smithson Tennant
78 Platinum Pt 195.08 Transition metals 1557 Julius Scaliger
79 Gold Au 196.9665 Transition metals prehistoric unknown
80 Mercury Hg 200.59 Transition metals prehistoric unknown
81 Thallium Tl 204.3833 Other metals 1861 William Crookes (isolated by William Crookes and Claude August Lamy, independently of each other, in 1862)
82 Lead Pb 207.2 Other metals prehistoric unknown
83 Bismuth Bi 208.9804 Other metals prehistoric unknown
84 Polonium Po (209) Other metals 1898 Marie and Pierre Curie
85 Astatine At (210) Halogens 1940 Dale R. Corson, K. R. MacKenzie, and Emilio Segrè
86 Radon Rn (222) Noble gases 1900 Friedrich Dorn
87 Francium Fr (223) Alkali metals 1939 Marguérite Perey
88 Radium Ra (226) Alkaline earth metals 1898 Marie Curie
89 Actinium Ac (227) Actinide series 1899 André Debierne
90 Thorium Th 232.0381 Actinide series 1828 Jöns Berzelius
91 Protactinium Pa 231.036 Actinide series 1913 Kasimir Fajans and O. Göhring
92 Uranium U 238.0289 Actinide series 1789 Martin Klaproth (isolated by Eugène Péligot 1841)
93 Neptunium Np (237) Actinide series 1940 Edwin McMillan and Philip Abelson
94 Plutonium Pu (244) Actinide series 1940 Glenn Seaborg, Edwin McMillan, Joseph Kennedy, and Arthur Wahl
95 Americium Am 243 Actinide series 1944 Glenn Seaborg, Ralph James, Leon Morgan, and Albert Ghiorso
96 Curium Cm (247) Actinide series 1944 Glenn Seaborg, Ralph James, and Albert Ghiorso
97 Berkelium Bk (247) Actinide series 1949 Glenn Seaborg, Stanley Thompson, and Albert Ghiorso
98 Californium Cf (251) Actinide series 1950 Glenn Seaborg, Stanley Thompson, Kenneth Street, Jr., and Albert Ghiorso
99 Einsteinium Es (252) Actinide series 1952 Albert Ghiorso and coworkers
100 Fermium Fm (257) Actinide series 1955 Albert Ghiorso and coworkers
101 Mendelevium Md (258) Actinide series 1955 Albert Ghiorso, Bernard G. Harvey, Gregory Choppin, Stanley Thompson, and Glenn Seaborg
102 Nobelium No (259) Actinide series 1958 Albert Ghiorso, Torbjørn Sikkeland, J. R. Walton, and Glenn Seaborg
103 Lawrencium Lr (260) Transition metals 1961 Albert Ghiorso, Torbjørn Sikkeland, Almon Larsh, and Robert Latimer
104 Rutherfordium Rf (261) Transition metals 1969 claimed by U.S. scientist Albert Ghiorso and coworkers (disputed by Soviet workers)
105 Dubnium Db (262) Transition metals 1970 claimed by Albert Ghiorso and coworkers (disputed by Soviet workers)
106 Seaborgium Sg (266) Transition metals 1974 claimed by Georgii Flerov and coworkers, and independently by Albert Ghiorso and coworkers
107 Bohrium Bh (262) Transition metals 1976 Georgii Flerov and Yuri Oganessian (confirmed by German scientist Peter Armbruster and coworkers)
108 Hassium Hs (263) Transition metals 1984 Peter Armbruster and coworkers
109 Meitnerium Mt (268) Transition metals 1982 Peter Armbruster and coworkers
110 Darmstadtium Ds (271) Transition metals 1994 team at the Heavy-Ion Research Laboratory, Darmstadt, Germany
111 Roentgenium Rg (272) Transition metals 1994 team at the Heavy-Ion Research Laboratory, Darmstadt, Germany
112 Ununbium Uub (277) Transition metals 1996 team at the Heavy-Ion Research Laboratory, Darmstadt, Germany
114 Ununquadium Uuq (285) Other metals 1998 team at the Joint Institute for Nuclear Research, Dubna, Russia
116 Ununhexium Uuh (292) Other metals 2000 team at the Joint Institute for Nuclear Research, Dubna, Russia

Alternative versions


Some versions of the table show a dark stair-step line along the metalloids. Metals are to the left of the line and non-metals to the right.[2]

The layout of the periodic table demonstrates recurring ("periodic") chemical properties. Elements are listed in order of increasing atomic number (i.e., the number of protons in the atomic nucleus). Rows are arranged so that elements with similar properties fall into the same columns (groups or families). According to quantum mechanical theories of electron configuration within atoms, each row (period) in the table corresponded to the filling of a quantum shell of electrons. There are progressively longer periods further down the table, grouping the elements into s-, p-, d- and f-blocks to reflect their electron configuration.


In printed tables, each element is usually listed with its element symbol and atomic number; many versions of the table also list the element's atomic mass and other information, such as its abbreviated electron configuration, electronegativity and most common valence numbers.

As of 2010, the table contains 118 chemical elements whose discoveries have been confirmed. Ninety-four are found naturally on Earth, and the rest are synthetic elements that have been produced artificially in particle accelerators. Elements 43 (technetium), 61 (promethium) and all elements greater than 83 (bismuth), beginning with 84 (polonium) have no stable isotopes. The atomic mass of each of these element's isotope having the longest half-life is typically reported on periodic tables with parentheses.[3] Isotopes of elements 43, 61, 93 (neptunium) and 94 (plutonium), first discovered synthetically, have since been discovered in trace amounts on Earth as products of natural radioactive decay processes.

The primary determinant of an element's chemical properties is its electron configuration, particularly the valence shell electrons. For instance, any atoms with four valence electrons occupying p orbitals will exhibit some similarity. The type of orbital in which the atom's outermost electrons reside determines the "block" to which it belongs. The number of valence shell electrons determines the family, or group, to which the element belongs.

Subshell S G F D P
Period
1 1s
2 2s 2p
3 3s 3p
4 4s 3d 4p
5 5s 4d 5p
6 6s 4f 5d 6p
7 7s 5f 6d 7p
8 8s 5g 6f 7d 8p
The total number of electron shells an atom has determines the period to which it belongs. Each shell is divided into different subshells, which as atomic number increases are filled in roughly this order (the Aufbau principle) (see table). Hence the structure of the table. Since the outermost electrons determine chemical properties, those with the same number of valence electrons are grouped together.

Progressing through a group from lightest element to heaviest element, the outer-shell electrons (those most readily accessible for participation in chemical reactions) are all in the same type of orbital, with a similar shape, but with increasingly higher energy and average distance from the nucleus. For instance, the outer-shell (or "valence") electrons of the first group, headed by hydrogen, all have one electron in an s orbital. In hydrogen, that s orbital is in the lowest possible energy state of any atom, the first-shell orbital (and represented by hydrogen's position in the first period of the table). In francium, the heaviest element of the group, the outer-shell electron is in the seventh-shell orbital, significantly further out on average from the nucleus than those electrons filling all the shells below it in energy. As another example, both carbon and lead have four electrons in their outer shell orbitals.

Note that as atomic number (i.e., charge on the atomic nucleus) increases, this leads to greater spin-orbit coupling between the nucleus and the electrons, reducing the validity of the quantum mechanical orbital approximation model, which considers each atomic orbital as a separate entity.

The elements ununtrium, ununquadium, ununpentium, etc. are elements that have been discovered, but so far have not received a trivial name yet. There is a system for naming them temporarily.

Classification
Groups


A group or family is a vertical column in the periodic table. Groups are considered the most important method of classifying the elements. In some groups, the elements have very similar properties and exhibit a clear trend in properties down the group. These groups tend to be given trivial (unsystematic) names, e.g., the alkali metals, alkaline earth metals, halogens, pnictogens, chalcogens, and noble gases. Some other groups in the periodic table display fewer similarities and/or vertical trends (for example Group 14), and these have no trivial names and are referred to simply by their group numbers.




Periods





A period is a horizontal row in the periodic table. Although groups are the most common way of classifying elements, there are some regions of the periodic table where the horizontal trends and similarities in properties are more significant than vertical group trends. This can be true in the d-block (or "transition metals"), and especially for the f-block, where the lanthanides and actinides form two substantial horizontal series of elements.

Main article: Periodic table block

Because of the importance of the outermost shell, the different regions of the periodic table are sometimes referred to as periodic table blocks, named according to the subshell in which the "last" electron resides. The s-block comprises the first two groups (alkali metals and alkaline earth metals) as well as hydrogen and helium. The p-block comprises the last six groups (groups 13 through 18) and contains, among others, all of the semimetals. The d-block comprises groups 3 through 12 and contains all of the transition metals. The f-block, usually offset below the rest of the periodic table, comprises the rare earth metals.

Other

The chemical elements are also grouped together in other ways. Some of these groupings are often illustrated on the periodic table, such as transition metals, poor metals, and metalloids. Other informal groupings exist, such as the platinum group and the noble metals.

Periodicity of chemical properties

The main value of the periodic table is the ability to predict the chemical properties of an element based on its location on the table. It should be noted that the properties vary differently when moving vertically along the columns of the table than when moving horizontally along the rows.
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