The standard atomic weights of nineteen elements have been changed as the result of cooperative research supported by the U.S. Geological Survey and the International Union of Pure and Applied Chemistry (IUPAC). The new values were announced at the August 2013 meeting of the IUPAC Commission on Atomic Weights and Isotopic Abundances held in Gebze, Turkey.
The standard atomic weights of molybdenum, cadmium, selenium, and thorium have been changed based on recent determinations of terrestrial isotopic abundances. In addition, the standard atomic weights of 15 elements have been revised based on a new assessment of their atomic masses by the International Union of Pure and Applied Physics.
Changes in standard atomic weights:
molybdenum: from 95.96(2) to 95.95(1)
beryllium: from 9.012 182(3) to 9.012 1831(5)
These changes will be published in 2014 by IUPAC in the journal Pure and Applied Chemistry as a new "Table of Standard Atomic Weights 2013."
Currently, the revised values for atomic weights can be found on-line in the Table of Standard Atomic Weights (xls) at the website of the Commission on Isotopic Abundances and Atomic Weights.
The importance of determining precise atomic weights has long been recognized. As far back as 1882, Frank W. Clarke, then a professor at the University of Cincinnati, prepared a table of atomic weights for use in science, industry, and trade. He carried on this work as Chief Chemist of the USGS (1883-1924). Clarke was a founder of the American Chemical Society and a member of the National Academy of Sciences. Recently, IUPAC has overseen the periodic evaluation and dissemination of atomic-weight values.
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