Atomic Weights

Atomic Weight

[2016. Scaled to an atomic weight of 12 for carbon-12 (12C), where 12C is a neutral atom in its nuclear and electronic ground state, having the result that atomic weight values are dimensionless.]

The atomic weights of many elements are not invariant, but depend on the origin and treatment of the material. The standard atomic weights apply to elements of natural terrestrial origin. Although the atomic weights of some elements in some naturally occurring materials may differ from given values because of a variation in the mole fractions of an element’s stable isotopes, the last significant figure of each tabulated value is considered reliable to ±1 except for zinc, which is ±2. For twelve of these elements, both a conventional atomic weight and an atomic weight interval is given with the symbol [a, b] to denote the set of atomic weight values in normal materials; thus, a ≤ atomic weight ≤ b. For lithium, the conventional atomic weight is only three digits because of the large variation found in lithium-bearing reagents.


Atomic Number Element Name Symbol Standard Atomic Weight Conventional Atomic Weight
1 hydrogen H [1.007, 1.009] 1.008
2 helium He 4.003
3 lithium Li [6.938, 6.997] 6.94
4 beryllium Be 9.012
5 boron B [10.80, 10.83] 10.81
6 carbon C [12.00, 12.02] 12.01
7 nitrogen N [14.00, 14.01] 14.01
8 oxygen O [15.99, 16.00] 16.00
9 fluorine F 19.00
10 neon Ne 20.18
11 sodium Na 22.99
12 magnesium Mg [24.30, 24.31] 24.31
13 aluminium (aluminum) Al 26.98
14 silicon Si [28.08, 28.09] 28.09
15 phosphorus P 30.97
16 sulfur S [32.05, 32.08] 32.06
17 chlorine Cl [35.44, 35.46] 35.45
18 argon Ar 39.95
19 potassium K 39.10
20 calcium Ca 40.08
21 scandium Sc 44.96
22 titanium Ti 47.87
23 vanadium V 50.94
24 chromium Cr 52.00
25 manganese Mn 54.94
26 iron Fe 55.85
27 cobalt Co 58.93
28 nickel Ni 58.69
29 copper Cu 63.55
30 zinc Zn 65.38
31 gallium Ga 69.72
32 germanium Ge 72.63
33 arsenic As 74.92
34 selenium Se 78.97
35 bromine Br [79.90, 79.91] 79.90
36 krypton Kr 83.80
37 rubidium Rb 85.47
38 strontium Sr 87.62
39 yttrium Y 88.91
40 zirconium Zr 91.22
41 niobium Nb 92.91
42 molybdenum Mo 95.95
43 technetium* Tc
44 ruthenium Ru 101.1
45 rhodium Rh 102.9
46 palladium Pd 106.4
47 silver Ag 107.9
48 cadmium Cd 112.4
49 indium In 114.8
50 tin Sn 118.7
51 antimony Sb 121.8
52 tellurium Te 127.6
53 iodine I 126.9
54 xenon Xe 131.3
55 caesium (cesium) Cs 132.9
56 barium Ba 137.3
57 lanthanum La 138.9
58 cerium Ce 140.1
59 praseodymium Pr 140.9
60 neodymium Nd 144.2
61 promethium* Pm
62 samarium Sm 150.4
63 europium Eu 152.0
64 gadolinium Gd 157.3
65 terbium Tb 158.9
66 dysprosium Dy 162.5
67 holmium Ho 164.9
68 erbium Er 167.3
69 thulium Tm 168.9
70 ytterbium Yb 173.1
71 lutetium Lu 175.0
72 hafnium Hf 178.5
73 tantalum Ta 180.9
74 tungsten W 183.8
75 rhenium Re 186.2
76 osmium Os 190.2
77 iridium Ir 192.2
78 platinum Pt 195.1
79 gold Au 197.0
80 mercury Hg 200.6
81 thallium Tl [204.3, 204.4] 204.4
82 lead Pb 207.2
83 bismuth* Bi 209.0
84 polonium* Po
85 astatine* At
86 radon* Rn
87 francium* Fr
88 radium* Ra
89 actinium* Ac
90 thorium* Th 232.0
91 protactinium* Pa 231.0
92 uranium* U 238.0
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 nihonium* Nh
114 flerovium* Fl
115 moscovium* Mc
116 livermorium* Lv
117 tennessine* Ts
118 oganesson* Og
*Element has no stable isotopes, only radioactive isotopes, and an atomic weight cannot be determined. However, four such elements (Bi, Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these a standard atomic weight is tabulated.

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Chemistry 109 Fall 2021 by John Moore, Jia Zhou, and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.