Current Table of Standard Atomic Weights Abridged to Five Significant Digits

Atomic weights of the elements 2009, Wieser, M. E., and Coplen, T. B.,  Pure Appl. Chem., 83, 359–396 (2011).   [full text - pdf 1.8 MB] {TSAW 2009.pdf}

Table of Standard Atomic Weights Abridged to Five Significant Digits
[Using A_r(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 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 atomic-weight 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.

*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.