Standard Atomic Weights Not Standard

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Atomic Weights Changed for Nineteen Chemical Elements

Posted: 24 Sep 2013 06:36 AM PDT

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) cadmium: from 112.411(8) to 112.414(4) selenium: from 78.96(3) to 78.971(8) thorium: from 232.038 06(2) to 232.0377(4)

beryllium: from 9.012 182(3) to 9.012 1831(5) fluorine: from 18.998 4032(5) to 18.998 403 163(6) aluminium (aluminum): from 26.981 5386(8) to 26.981 5385(7) phosphorus: from 30.973 762(2) to 30.973 761 998(5) scandium: from 44.955 912(6) to 44.955 908(5) manganese: from 54.938 045(5) to 54.938 044(3) cobalt: from 58.933 195(5) to 58.933 194(4) arsenic: from 74.921 60(2) to 74.921 595(6) yttrium: from 88.905 85(2) to 88.905 84(2) niobium: from 92.906 38(2) to 92.906 37(2) caesium (cesium): from 132.905 4519(2) to 132.905 451 96(6) praseodymium: from 140.907 65(2) to 140.907 66(2) holmium: from 164.930 32(2) to 164.930 33(2) thulium: from 168.934 21(2) to 168.934 22(2) gold: from 196.966 569(4) to 196.966 569(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.