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. (3) our ability to precisely determine the atomic masses of the isotopes.Įlements in the first category are distinguished by an interval standard atomic weight. (2) our ability to determine the isotopic abundances, and ![]() (1) well-documented natural variations of isotopic abundances, There are three broad groups of elements depending on what is the main cause of the uncertainty of their standard atomic weights: The most recent Standard Atomic Weights are presented in this Table and they are based on the "Atomic Weights 2021" report. 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. Substantial deviations in atomic weight of the element from that given in the table can occur. M Modified isotopic compositions may be found in commercially available material because the material has been subjected to some undisclosed or inadvertent isotopic fractionation. ![]() The difference between the atomic weight of the element in such materials and that given in the table may exceed the stated uncertainty. ![]() G Geological and biological materials are known in which the element has an isotopic composition outside the limits for normal material. The most recent definitive table of the standard atomic weights. Since 1902, the Commission regularly publishes critical evaluation of atomic weights of elements and below is Standard atomic weights are CIAAW recommended values for atomic weights applicable to all normal materials.
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