Standard Atomic Weights 2009
[Scaled to Ar(12C) ⫽ 12]
The atomic weights of many elements are not invariant but depend on the origin and treatment of the material. The standard values of Ar(E) and the uncertainties (in parentheses,
following the last significant figure to which they are attributed) apply to elements of natural terrestrial origin. The footnotes to this table elaborate the types of variation which may
occur for individual elements and that may be larger than the listed uncertainties of values of Ar(E). Names of elements with atomic number 113 to 118 are provisional.
Atomic Atomic
Name Symbol Number Atomic Weight Footnotes Name Symbol Number Atomic Weight Footnotes
*
Actinium Ac 89 Molybdenum Mo 42 95.96(2) g
Aluminum Al 13 26.981 5386(8) Neodymium Nd 60 144.242(3) g
Americium* Am 95 Neon Ne 10 20.1797(6) g, m
Antimony Sb 51 121.760(1) g Neptunium* Np 93
Argon Ar 18 39.948(1) g, r Nickel Ni 28 58.6934(4)
Arsenic As 33 74.921 60(2) Niobium Nb 41 92.906 38(2)
Astatine* At 85 Nitrogen N 7 [14.006 43; 14.007 28]
Barium Ba 56 137.327(7) Nobelium* No 102
Berkelium* Bk 97 Osmium Os 76 190.23(3) g
Beryllium Be 4 9.012 182(3) Oxygen O 8 [15.999 03; 15.999 77]
Bismuth Bi 83 208.980 40(1 ) Palladium Pd 46 106.42(1) g
Bohrium* Bh 107 Phosphorus P 15 30.973 762(2)
Boron B 5 [10.806; 10.821] m Platinum Pt 78 195.084(9)
Bromine Br 35 79.904(1) Plutonium* Pu 94
Cadmium Cd 48 112.411(8) g Polonium* Po 84
Calcium Ca 20 40.078(4) g Potassium K 19 39.0983(1)
Californium* Cf 98 Praseodymium Pr 59 140.907 65(2)
Carbon C 6 [12.0096; 12.0116] Promethium* Pm 61
Cerium Ce 58 140.116(1) g Protactinium* Pa 91 231.035 88(2)
Cesium Cs 55 132.905 4519(2) Radium* Ra 88
Chlorine Cl 17 [35.446; 35.457] m Radon* Rn 86
Chromium Cr 24 51.9961(6) Roentgenium* Rg 111
Cobalt Co 27 58.933 195(5) Rhenium Re 75 186.207(1)
Copernicium* Cn 112 Rhodium Rh 45 102.905 50(2)
Copper Cu 29 63.546(3) r Rubidium Rb 37 85.4678(3) g
Curium* Cm 96 Ruthenium Ru 44 101.07(2) g
Darmstadtium Ds 110 Rutherfordium* Rf 104
Dubnium* Db 105 Samarium Sm 62 150.36(2) g
Dysprosium Dy 66 162.500(1) g Scandium Sc 21 44.955 912(6)
Einsteinium* Es 99 Seaborgium* Sg 106
Erbium Er 68 167.259(3) g Selenium Se 34 78.96(3) r
Europium Eu 63 151.964(1) g Silicon Si 14 [28.084; 28.086]
Fermium* Fm 100 Silver Ag 47 107.8682(2) g
Fluorine F 9 18.998 4032(5) Sodium Na 11 22.989 769 28(2)
Francium* Fr 87 Strontium Sr 38 87.62(1) g, r
Gadolinium Gd 64 157.25(3) g Sulfur S 16 [32.059; 32.076]
Gallium Ga 31 69.723(1) Tantalum Ta 73 180.947 88(2)
Germanium Ge 32 72.63(1) Technetium* Tc 43
Gold Au 79 196.966 569(4) Tellurium Te 52 127.60(3) g
Hafnium Hf 72 178.49(2) Terbium Tb 65 158.925 35(2)
Hassium* Hs 108 Thallium Tl 81 [204.382; 204.385]
Helium He 2 4.002 602(2) g, r Thorium* Th 90 232.03806(2) g
Holmium Ho 67 164.930 32(2) Thulium Tm 69 168.934 21(2)
Hydrogen H 1 [1.007 84; 1008 11] m Tin Sn 50 118.710(7) g
Indium In 49 114.818(3) Titanium Ti 22 47.867(1)
Iodine I 53 126.904 47(3) Tungsten W 74 183.84(1)
Iridium Ir 77 192.217(3) Ununhexium* Uuh 116
Iron Fe 26 55.845(2) Ununoctium* Uuo 118
Krypton Kr 36 83.798(2) g, m Ununpentium* Uup 115
Lanthanum La 57 138.905 47(7) g Ununquadium* Uuq 114
Lawrencium* Lr 103 Ununtrium* Uut 113
Lead Pb 82 207.2(1) g, r Uranium* U 92 238.02891(3) g, m
Lithium Li 3 [6.938; 6.997] m Vanadium V 23 50.9415(1)
Lutetium Lu 71 174.9668(1) g Xenon Xe 54 131.293(6) g, m
Magnesium Mg 12 24.3050(6) Ytterbium Yb 70 173.054(5) g
Manganese Mn 25 54.938 045(5) Yttrium Y 39 88.905 85(2)
Meitnerium* Mt 109 Zinc Zn 30 65.38(2) r
Mendelevium* Md 101 Zirconium Zr 40 91.224(2) g
Mercury Hg 80 200.59(2)
* Element has no stable nuclides.
g Geological specimens are known in which the element has an isotopic composition outside the limits for normal material. The difference between the atomic weight of the element
in such specimens and that given in the Table may exceed the stated uncertainty.
m Modified isotopic compositions may be found in commercially available material because it has been subjected to an undisclosed or inadvertent isotopic fractionation. Substantial
deviations in atomic weight of the element from that given in the table can occur.
r Range in isotopic composition of normal terrestrial material prevents a more precise Ar(E) being given; the tabulated Ar(E) value should be applicable to any normal material.
Source: INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY. 2010. Atomic weights of the elements, 2009. Pure Appl. Chem. 83:359.
