Logo of the Atomic Weights Commission Logo of the International Union of Pure and Applied Chemistry

Standard Atomic Weights

Standard atomic weights are CIAAW recommended values for atomic weights applicable to all normal materials. Since 1902, the Commission regularly publishes critical evaluation of atomic weights of elements and below is the most recent definitive table of the standard atomic weights.

Standard Atomic Weights 2017

Z Symbol Element Standard Atomic Weight Notes
1Hhydrogen  [1.007 84, 1.008 11]m
2Hehelium   4.002 602(2)g r
3Lilithium  [6.938, 6.997]m
4Beberyllium   9.012 1831(5)
5Bboron [10.806, 10.821]m
6Ccarbon [12.0096, 12.0116]
7Nnitrogen [14.006 43, 14.007 28]m
8Ooxygen [15.999 03, 15.999 77]m
9Ffluorine  18.998 403 163(6)
10Neneon  20.1797(6)g m
11Nasodium  22.989 769 28(2)
12Mgmagnesium [24.304, 24.307]
13Alaluminium  26.981 5384(3)
14Sisilicon [28.084, 28.086]
15Pphosphorus  30.973 761 998(5)
16Ssulfur [32.059, 32.076]
17Clchlorine [35.446, 35.457]m
18Arargon [39.792, 39.963]g r
19Kpotassium  39.0983(1)
20Cacalcium  40.078(4)g
21Scscandium  44.955 908(5)
22Tititanium  47.867(1)
23Vvanadium  50.9415(1)
24Crchromium  51.9961(6)
25Mnmanganese  54.938 043(2)
26Feiron  55.845(2)
27Cocobalt  58.933 194(3)
28Ninickel  58.6934(4)r
29Cucopper  63.546(3)r
30Znzinc  65.38(2)r
31Gagallium  69.723(1)
32Gegermanium  72.630(8)
33Asarsenic  74.921 595(6)
34Seselenium  78.971(8)r
35Brbromine [79.901, 79.907]
36Krkrypton  83.798(2)g m
37Rbrubidium  85.4678(3)g
38Srstrontium  87.62(1)g r
39Yyttrium  88.905 84(1)
40Zrzirconium  91.224(2)g
41Nbniobium  92.906 37(1)
42Momolybdenum  95.95(1)g
43Tctechnetium —
44Ruruthenium 101.07(2)g
45Rhrhodium 102.905 49(2)
46Pdpalladium 106.42(1)g
47Agsilver 107.8682(2)g
48Cdcadmium 112.414(4)g
49Inindium 114.818(1)
50Sntin 118.710(7)g
51Sbantimony 121.760(1)g
52Tetellurium 127.60(3)g
53Iiodine 126.904 47(3)
54Xexenon 131.293(6)g m
55Cscaesium 132.905 451 96(6)
56Babarium 137.327(7)
57Lalanthanum 138.905 47(7)g
58Cecerium 140.116(1)g
59Prpraseodymium 140.907 66(1)
60Ndneodymium 144.242(3)g
61Pmpromethium —
62Smsamarium 150.36(2)g
63Eueuropium 151.964(1)g
64Gdgadolinium 157.25(3)g
65Tbterbium 158.925 354(8)
66Dydysprosium 162.500(1)g
67Hoholmium 164.930 328(7)
68Ererbium 167.259(3)g
69Tmthulium 168.934 218(6)
70Ybytterbium 173.045(10)g
71Lulutetium 174.9668(1)g
72Hfhafnium 178.49(2)
73Tatantalum 180.947 88(2)
74Wtungsten 183.84(1)
75Rerhenium 186.207(1)
76Ososmium 190.23(3)g
77Iriridium 192.217(2)
78Ptplatinum 195.084(9)
79Augold 196.966 570(4)
80Hgmercury 200.592(3)
81Tlthallium[204.382, 204.385]
82Pblead 207.2(1)g r
83Bibismuth 208.980 40(1)
84Popolonium —
85Atastatine —
86Rnradon —
87Frfrancium —
88Raradium —
89Acactinium —
90Ththorium 232.0377(4)
91Paprotactinium 231.035 88(1)
92Uuranium 238.028 91(3)g m
93Npneptunium —
94Puplutonium —
95Amamericium —
96Cmcurium —
97Bkberkelium —
98Cfcalifornium —
99Eseinsteinium —
100Fmfermium —
101Mdmendelevium —
102Nonobelium —
103Lrlawrencium —
104Rfrutherfordium —
105Dbdubnium —
106Sgseaborgium —
107Bhbohrium —
108Hshassium —
109Mtmeitnerium —
110Dsdarmstadtium —
111Rgroentgenium  —
112Cncopernicium —
113Nhnihonium —
114Flflerovium —
115Mcmoscovium —
116Lvlivermorium —
117Tstennessine —
118Ogoganesson —
Z Symbol Element Standard Atomic Weight Notes

Footnotes      Back to Top
Geological materials 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 materials and that given in the table may exceed the stated uncertainty.
Modified isotopic compositions may be found in commercially available material because the material 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.
Range in isotopic composition of normal terrestrial material prevents a more precise standard atomic weight being given; the tabulated atomic-weight value and uncertainty should be applicable to normal materials.

Citation

The most recent Standard Atomic Weights are presented in this Table and they are based on the "Atomic Weights 2013" report and on the subsequent revisions that were made by the CIAAW in 2015 and in 2017. The IUPAC Technical Report "Atomic weights of the elements 2017" will be published in the Pure and Applied Chemistry.

This Table can be cited as follows:
CIAAW. Atomic weights of the elements 2017. Available online at www.ciaaw.org.

There are three broad groups of elements depending on what is the main cause of the uncertainty of their standard atomic weights:
  (1) well-documented natural variations of isotopic abundances,
  (2) our ability to determine the isotopic abundances, and
  (3) our ability to precisely determine the atomic masses of the isotopes.
Elements in the first category are distinguished by an interval standard atomic weight.

The reported uncertainties of the standard atomic weights are such that the atomic-weight values of normal materials are expected to lie in the given interval with great certitude. For instance, the standard atomic weight of argon, [39.792, 39.963], indicates that atomic-weight values of argon in normal materials are expected to be from 39.792 to 39.963. For iridium, the standard atomic weight 192.217(2) indicates that atomic-weight values of iridium in normal materials are expected to be from 192.215 to 192.219.
For more information on the interpretation of the uncertainty please consult the recent IUPAC Technical Report by Possolo et al.