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.
| Z | Symbol | Element | Standard Atomic Weight | Notes |
|---|---|---|---|---|
| 1 | H | hydrogen | [1.007 84, 1.008 11] | m |
| 2 | He | helium | 4.002 602(2) | g r |
| 3 | Li | lithium | [6.938, 6.997] | m |
| 4 | Be | beryllium | 9.012 1831(5) | |
| 5 | B | boron | [10.806, 10.821] | m |
| 6 | C | carbon | [12.0096, 12.0116] | |
| 7 | N | nitrogen | [14.006 43, 14.007 28] | m |
| 8 | O | oxygen | [15.999 03, 15.999 77] | m |
| 9 | F | fluorine | 18.998 403 163(6) | |
| 10 | Ne | neon | 20.1797(6) | g m |
| 11 | Na | sodium | 22.989 769 28(2) | |
| 12 | Mg | magnesium | [24.304, 24.307] | |
| 13 | Al | aluminium | 26.981 5384(3) | |
| 14 | Si | silicon | [28.084, 28.086] | |
| 15 | P | phosphorus | 30.973 761 998(5) | |
| 16 | S | sulfur | [32.059, 32.076] | |
| 17 | Cl | chlorine | [35.446, 35.457] | m |
| 18 | Ar | argon | [39.792, 39.963] | g r |
| 19 | K | potassium | 39.0983(1) | |
| 20 | Ca | calcium | 40.078(4) | g |
| 21 | Sc | scandium | 44.955 908(5) | |
| 22 | Ti | titanium | 47.867(1) | |
| 23 | V | vanadium | 50.9415(1) | |
| 24 | Cr | chromium | 51.9961(6) | |
| 25 | Mn | manganese | 54.938 043(2) | |
| 26 | Fe | iron | 55.845(2) | |
| 27 | Co | cobalt | 58.933 194(3) | |
| 28 | Ni | nickel | 58.6934(4) | r |
| 29 | Cu | copper | 63.546(3) | r |
| 30 | Zn | zinc | 65.38(2) | r |
| 31 | Ga | gallium | 69.723(1) | |
| 32 | Ge | germanium | 72.630(8) | |
| 33 | As | arsenic | 74.921 595(6) | |
| 34 | Se | selenium | 78.971(8) | r |
| 35 | Br | bromine | [79.901, 79.907] | |
| 36 | Kr | krypton | 83.798(2) | g m |
| 37 | Rb | rubidium | 85.4678(3) | g |
| 38 | Sr | strontium | 87.62(1) | g r |
| 39 | Y | yttrium | 88.905 84(1) | |
| 40 | Zr | zirconium | 91.224(2) | g |
| 41 | Nb | niobium | 92.906 37(1) | |
| 42 | Mo | molybdenum | 95.95(1) | g |
| 43 | Tc | technetium | — | |
| 44 | Ru | ruthenium | 101.07(2) | g |
| 45 | Rh | rhodium | 102.905 49(2) | |
| 46 | Pd | palladium | 106.42(1) | g |
| 47 | Ag | silver | 107.8682(2) | g |
| 48 | Cd | cadmium | 112.414(4) | g |
| 49 | In | indium | 114.818(1) | |
| 50 | Sn | tin | 118.710(7) | g |
| 51 | Sb | antimony | 121.760(1) | g |
| 52 | Te | tellurium | 127.60(3) | g |
| 53 | I | iodine | 126.904 47(3) | |
| 54 | Xe | xenon | 131.293(6) | g m |
| 55 | Cs | caesium | 132.905 451 96(6) | |
| 56 | Ba | barium | 137.327(7) | |
| 57 | La | lanthanum | 138.905 47(7) | g |
| 58 | Ce | cerium | 140.116(1) | g |
| 59 | Pr | praseodymium | 140.907 66(1) | |
| 60 | Nd | neodymium | 144.242(3) | g |
| 61 | Pm | promethium | — | |
| 62 | Sm | samarium | 150.36(2) | g |
| 63 | Eu | europium | 151.964(1) | g |
| 64 | Gd | gadolinium | 157.25(3) | g |
| 65 | Tb | terbium | 158.925 354(8) | |
| 66 | Dy | dysprosium | 162.500(1) | g |
| 67 | Ho | holmium | 164.930 328(7) | |
| 68 | Er | erbium | 167.259(3) | g |
| 69 | Tm | thulium | 168.934 218(6) | |
| 70 | Yb | ytterbium | 173.045(10) | g |
| 71 | Lu | lutetium | 174.9668(1) | g |
| 72 | Hf | hafnium | 178.49(2) | |
| 73 | Ta | tantalum | 180.947 88(2) | |
| 74 | W | tungsten | 183.84(1) | |
| 75 | Re | rhenium | 186.207(1) | |
| 76 | Os | osmium | 190.23(3) | g |
| 77 | Ir | iridium | 192.217(2) | |
| 78 | Pt | platinum | 195.084(9) | |
| 79 | Au | gold | 196.966 570(4) | |
| 80 | Hg | mercury | 200.592(3) | |
| 81 | Tl | thallium | [204.382, 204.385] | |
| 82 | Pb | lead | 207.2(1) | g r |
| 83 | Bi | bismuth | 208.980 40(1) | |
| 84 | Po | polonium | — | |
| 85 | At | astatine | — | |
| 86 | Rn | radon | — | |
| 87 | Fr | francium | — | |
| 88 | Ra | radium | — | |
| 89 | Ac | actinium | — | |
| 90 | Th | thorium | 232.0377(4) | |
| 91 | Pa | protactinium | 231.035 88(1) | |
| 92 | U | uranium | 238.028 91(3) | g m |
| 93 | Np | neptunium | — | |
| 94 | Pu | plutonium | — | |
| 95 | Am | americium | — | |
| 96 | Cm | curium | — | |
| 97 | Bk | berkelium | — | |
| 98 | Cf | californium | — | |
| 99 | Es | einsteinium | — | |
| 100 | Fm | fermium | — | |
| 101 | Md | mendelevium | — | |
| 102 | No | nobelium | — | |
| 103 | Lr | lawrencium | — | |
| 104 | Rf | rutherfordium | — | |
| 105 | Db | dubnium | — | |
| 106 | Sg | seaborgium | — | |
| 107 | Bh | bohrium | — | |
| 108 | Hs | hassium | — | |
| 109 | Mt | meitnerium | — | |
| 110 | Ds | darmstadtium | — | |
| 111 | Rg | roentgenium | — | |
| 112 | Cn | copernicium | — | |
| 113 | Nh | nihonium | — | |
| 114 | Fl | flerovium | — | |
| 115 | Mc | moscovium | — | |
| 116 | Lv | livermorium | — | |
| 117 | Ts | tennessine | — | |
| 118 | Og | oganesson | — | |
| Z | Symbol | Element | Standard Atomic Weight | Notes |
g 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.
m 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.
r 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.


