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The biennial review of atomic-weight determinations and other cognate data has resulted in changes for the standard atomic weights of 11 elements. Many atomic weights are not constants of nature, but depend upon the physical, chemical, and nuclear history of the material. The standard atomic weights of 10 elements having two or more stable isotopes have been changed to reflect this variability of atomic-weight values in natural terrestrial materials. To emphasize the fact that these standard atomic weights are not constants of nature, each atomic-weight value is expressed as an interval. The interval is used together with the symbol [a; b] to denote the set of atomic-weight values, Ar(E), of element E in normal...