This table of atomic weights includes the changes made through 2020 by the International Union of Pure and Applied Chemistry (IUPAC) Commission on Isotopic Abundances and Atomic Weights (CIAAW) (Refs. 1, 5-9).
IUPAC made a significant policy change in 2009 (Refs. 2-4). Each atomic weight had previously been given as a single value with an uncertainty that took into account both the measurement uncertainty and the variation in isotopic abundance in samples of the element from different terrestrial sources. For a variety of reasons (Ref. 3), this fails to give complete information on the natural variability in isotopic abundance of several elements. Therefore, beginning in 2009 the recommendations expressed the atomic weights of several elements as intervals rather than single numbers plus uncertainties. The symbol for these intervals is [a, b], where a is the lower bound of values found in normal materials and b the upper bound. For the other elements in the table, a single recommended atomic weight value is given; the number in parentheses following the value gives the uncertainty in the last digit. CIAAW frequently makes adjustments based on emerging measurement results on samples from various geographical locations. For example, atomic weights for hafnium (2019) and lead (2020) have recently been revised.
Table 1 gives the atomic weights of the elements listed in alphabetical order by name. Table 2 gives conventional reference atomic weights for 14 elements whose entries in Table 1 are intervals rather than single numbers. These conventional reference values are suggested for use on samples of unspecified origin and for calculation of molecular weights in tables intended to be broadly applicable. They have been selected such that most or all natural terrestrial atomic-weight variations are covered in an interval of plus or minus one in the last digit. It should be emphasized that the conventional values are not simply midpoints of the intervals, but rather represent the best judgment of the data evaluators.
Element | Symbol | CAS Reg. No. | Atomic number | Atomic weight |
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Actiniumu | Ac | 7440-34-8 | 89 | |
Aluminum | Al | 7429-90-5 | 13 | 26.9815384(3) |
Americiumu | Am | 7440-35-9 | 95 | |
Antimonyg | Sb | 7440-36-0 | 51 | 121.760(1) |
Argong,r | Ar | 7440-37-1 | 18 | [39.792, 39.963] |
Arsenic | As | 7440-38-2 | 33 | 74.921595(6) |
Astatineu | At | 7440-68-8 | 85 | |
Barium | Ba | 7440-39-3 | 56 | 137.327(7) |
Berkeliumu | Bk | 7440-40-6 | 97 | |
Beryllium | Be | 7440-41-7 | 4 | 9.0121831(5) |
Bismuthu | Bi | 7440-69-9 | 83 | 208.98040(1) |
Bohriumu | Bh | 54037-14-8 | 107 | |
Boronm,* | B | 7440-42-8 | 5 | [10.806, 10.821] |
Bromine* | Br | 7726-95-6 | 35 | [79.901, 79.907] |
Cadmiumg | Cd | 7440-43-9 | 48 | 112.414(4) |
Calciumg | Ca | 7440-70-2 | 20 | 40.078(4) |
Californiumu | Cf | 7440-71-3 | 98 | |
Carbon* | C | 7440-44-0 | 6 | [12.0096, 12.0116] |
Ceriumg | Ce | 7440-45-1 | 58 | 140.116(1) |
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