HOW Good Are the Standard Atomic Weights?

Abstract

Science has not stood still since 1961. Based mostly on more searching analysis and better mass spectrometry, the 1983 IUPAC Table of Atomic Weights contains a good deal of better data than the 1961 table. It is now well recognized that atomic weights are not all constants of nature. It is thus appropriate to assign an atomic weight to a given sample. Accordingly IUPAC feels free to define the standard atomic weight of an element as referring to our best knowledge of the atomic weight of an element in normal natural terrestrial sources. Its uncertainty is implied by the precision of its tabulated value and arises from experimental uncertainty and variability of isotopic composition in such sources. The IUPAC table is now called the Table of the Standard Atomic Weights, as recommended by the IUPAC Commission now called Commission on Atomic Weights and Isotopic Abundances (CAWIA). A subcommittee of CAWIA has just completed another element by element review of their atomic weights. How good are they now. In this simplified discussion the author is chiefly addressing chemical analysts, who perhaps place too much confidence in the accuracy to these values. 5 references, 4 tables.