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.

Standard Atomic Weights 2024

Footnotes      Back to Top
g Geological and biological 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 some 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 value and uncertainty should be applicable to normal material.

Citation

The most recent Standard Atomic Weights are presented in this Table and they are based on the "Atomic Weights 2021" report. Standard Atomic Weights of gadolinium, lutetium, and zirconium have been revised by the CIAAW in 2024 and these revisions are included in this table.

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. Thus, the standard atomic weight of argon, A_r°(Ar) = [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 A_r°(Ir) = 192.217 ± 0.002 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.