Current Table of Standard Atomic Weights in Order of Atomic Number

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 2009
[Using A_r(¹²C) = 12 as reference, where ¹²C is an unbound neutral atom in its nuclear and electronic ground state.]

The atomic weights, A_r(E), of many elements vary due to variations in the abundances of their isotopes in natural terrestrial materials. For 10 of these elements, an atomic-weight interval is given with the symbol [a; b] to denote the set of atomic-weight values in normal materials; thus, a ≤ A_r(E) ≤ b for element E. The symbols a and b denote the bounds of the interval [a; b]. Figures in this report display distributions of atomic-weight variation for specified elements. If a more accurate A_r(E) value is required, it must be determined for the specific material. For 74 elements, A_r(E) values and their decisional uncertainties (in parentheses, following the last significant figure to which they are attributed) are given. The footnotes to this table elaborate the types of variation that may occur for individual elements and that may be larger than the listed uncertainties of values of A_r(E) or may lie outside the values listed. Names of elements with atomic number 113 to 118 are provisional; they have been reported in the peer-reviewed, scientific literature, but they have not yet been named by IUPAC.

*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 A_r(E) being given; the tabulated A_r(E) value and uncertainty should be applicable to normal material.