Table 5 Standard atomic weights 2009 abridged to five significant digits.
[Using Ar(12C) = 12 as reference, where 12C is an unbound neutral atom in its nuclear and
electronic ground state.]
The atomic weights, Ar(E), of many elements vary due to variations in the abundances of
their isotopes in natural terrestrial materials. For 10 elements having two or more stable
isotopes, an atomic-weight interval is given with the symbol [a; b] to denote the set of
atomic-weight values in normal materials; thus, a ≤ Ar(E) ≤ b for element E. The symbols
a and b denote the lower and upper bounds of the interval [a; b], respectively. Atomic
weights are quoted here to five significant figures unless the dependable accuracy is further
limited by either the combined uncertainties of the best published atomic-weight
determinations or by the variability of isotopic composition in normal terrestrial
occurrences (the latter applies to the elements annotated r). Excluding values given as
atomic-weight intervals, the last significant figure of each tabulated value is considered
reliable to ±1 except when a larger single digit uncertainty is inserted in parentheses
following the atomic weight. Neither the highest nor the lowest actual atomic weight of any
normal sample is thought likely to differ from the tabulated values by more than one
assigned uncertainty. However, the tabulated values do not apply either to samples of highly
exceptional isotopic composition arising from most unusual geological occurrences (for
elements annotated g) or to those whose isotopic composition has been artificially altered.
Such might even be found in commerce without disclosure of that modification (for
elements annotated m). Elements with no stable isotope do not have an atomic weight, and
such entries have a blank in the atomic-weight column. However, three such elements (Th,
Pa and U) do have a characteristic terrestrial isotopic composition and for these an atomicweight
value is tabulated. For more detailed information, users should refer to the full
IUPAC Table of Standard Atomic Weights. Names of elements with atomic number 113 to
118 are provisional.
Order of atomic number
Atomic number Element name Symbol Atomic weight Footnotes
1 hydrogen H [1.0078; 1.0082] m
2 helium He 4.0026
3 lithium Li [6.938; 6.997] m
4 beryllium Be 9.0122
5 boron B [10.806; 10.821] m
6 carbon C [12.009; 12.012]
7 nitrogen N [14.006; 14.008]
8 oxygen O [15.999; 16.000]
9 fluorine F 18.998
10 neon Ne 20.180 m
11 sodium Na 22.990
12 magnesium Mg 24.305
13 aluminium (aluminum) Al 26.982
14 silicon Si [28.084; 28.086]
15 phosphorus P 30.974
16 sulfur S [32.059; 32.076]
17 chlorine Cl [35.446; 35.457] m
18 argon Ar 39.948 g r
19 potassium K 39.098 g
20 calcium Ca 40.078(4) g
21 scandium Sc 44.956
M. E. WIESER AND T. B. COPLEN
© 2010, IUPAC Pure Appl. Chem., Vol. xx, No. x, pp. 0000–0000, yyyy
32
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22 titanium Ti 47.867
23 vanadium V 50.942
24 chromium Cr 51.996
25 manganese Mn 54.938
26 iron Fe 55.845(2)
27 cobalt Co 58.933
28 nickel Ni 58.693 r
29 copper Cu 63.546(3) r
30 zinc Zn 65.38(2) r
31 gallium Ga 69.723
32 germanium Ge 72.63
33 arsenic As 74.922
34 selenium Se 78.96(3) r
35 bromine Br 79.904
36 krypton Kr 83.798(2) g m
37 rubidium Rb 85.468
38 strontium Sr 87.62 g r
39 yttrium Y 88.906
40 zirconium Zr 91.224(2) g
41 niobium Nb 92.906(2)
42 molybdenum Mo 95.96(2) g
43 technetium* Tc
44 ruthenium Ru 101.07(2) g
45 rhodium Rh 102.91
46 palladium Pd 106.42 g
47 silver Ag 107.87 g
48 cadmium Cd 112.41
49 indium In 114.82
50 tin Sn 118.71
51 antimony Sb 121.76 g
52 tellurium Te 127.60(3) g
53 iodine I 126.90
54 xenon Xe 131.29 g m
55 caesium (cesium) Cs 132.91
56 barium Ba 137.33
57 lanthanum La 138.91
58 cerium Ce 140.12 g
59 praseodymium Pr 140.91
60 neodymium Nd 144.24 g
61 promethium* Pm
62 samarium Sm 150.36(2) g
63 europium Eu 151.96 g
64 gadolinium Gd 157.25(3) g
65 terbium Tb 158.93
66 dysprosium Dy 162.50 g
67 holmium Ho 164.93
© 2010, IUPAC Pure Appl. Chem., Vol. xx, No. x, pp. 0000–0000, yyyy
Atomic weights of the elements 2009 33
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Table 5 (Continued).
Order of atomic number
Atomic number Element name Symbol Atomic weight Footnotes
68 erbium Er 167.26 g
69 thulium Tm 168.93
70 ytterbium Yb 173.05 g
71 lutetium Lu 174.97 g
72 hafnium Hf 178.49(2)
73 tantalum Ta 180.95
74 tungsten W 183.84
75 rhenium Re 186.21
76 osmium Os 190.23(3) g
77 iridium Ir 192.22
78 platinum Pt 195.08
79 gold Au 196.97
80 mercury Hg 200.59(2)
81 thallium Tl [204.38; 204.39]
82 lead Pb 207.2 g r
83 bismuth Bi 208.98
84 polonium* Po
85 astatine* At
86 radon* Rn
87 francium* Fr
88 radium* Ra
89 actinium* Ac
90 thorium* Th 232.04 g
91 protactinium* Pa 231.04
92 uranium* U 238.03 g m
93 neptunium* Np
94 plutonium* Pu
95 americium* Am
96 curium* Cm
97 berkelium* Bk
98 californium* Cf
99 einsteinium* Es
100 fermium* Fm
101 mendelevium* Md
102 nobelium* No
103 lawrencium* Lr
104 rutherfordium* Rf
105 dubnium* Db
106 seaborgium* Sg
107 bohrium* Bh
108 hassium* Hs
109 meitnerium* Mt
110 darmstadtium* Ds
111 roentgenium* Rg
112 copernicium * Cn
113 ununtrium* Uut
114 ununquadium* Uuq
115 ununpentium* Uup
M. E. WIESER AND T. B. COPLEN
© 2010, IUPAC Pure Appl. Chem., Vol. xx, No. x, pp. 0000–0000, yyyy
34
Table 5 (Continued).
Order of atomic number
Atomic number Element name Symbol Atomic weight Footnotes
(continues on next page)
116 ununhexium* Uuh
118 ununoctium* Uuo
*Element has no stable isotopes. One or more well-known isotopes are given in Table 3 with the
appropriate relative atomic mass and half-life. However, three such elements (Th, Pa, and U) do have
a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated.
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 and uncertainty should be applicable to normal material.
6. CONVENTIONAL ATOMIC-WEIGHT VALUES FOR SELECTED ELEMENTS
At its 2009 meeting in Vienna, the Commission reviewed recommendations from the task group of the
IUPAC project “Assessment of fundamental understanding of isotopic abundances and atomic weights
of the chemical elements” [8]. In addition to recommending that intervals be included in the 2009 Table
of Standard Atomic Weights for elements having two or more isotopes when the variation in atomic
weights in natural terrestrial occurrences exceeds that of the measurement uncertainty of the standard
atomic weight determined from a best measurement of isotopic abundances, the task group recognized
that some users may need a representative value for an element having an atomic-weight interval, such
as for trade and commerce. Conventional atomic-weight values are conventional quantity values [21],
and they are provided in Table 6 for these users. These conventional values have no uncertainty values
associated with them. They have been selected so that most or all natural terrestrial atomic-weight variation
is covered in an interval of plus or minus one in the last digit. For example, the conventional
atomic-weight value of boron, 10.81, is selected to account for boron-bearing materials with atomicweight
values between 10.80 and 10.82, which is the majority of boron-bearing substances (Fig. 5).
Table 6 Conventional atomic weights 2009.
[For users needing an atomic-weight value for an unspecified
sample, such as for trade and commerce, the following
conventional values are provided.]
Element name Symbol Atomic Reference
number atomic weight
boron B 5 10.81
carbon C 6 12.011
chlorine Cl 17 35.45
hydrogen H 1 1.008
lithium Li 3 6.94
nitrogen N 7 14.007
oxygen O 8 15.999
silicon Si 14 28.085
sulfur S 16 32.06
thallium Tl 81 204.38
© 2010, IUPAC Pure Appl. Chem., Vol. xx, No. x, pp. 0000–0000, yyyy
Atomic weights of the elements 2009 35
Table 5 (Continued).
Order of atomic number
Atomic number Element name Symbol Atomic weight Footnotes