New Format For Atomic Weights | December 20, 2010 Issue - Vol. 88 Issue 51 | Chemical & Engineering News
Volume 88 Issue 51 | p. 35 | Concentrates
Issue Date: December 20, 2010

New Format For Atomic Weights

Mass interval reflects natural variations in isotope abundances
Department: Science & Technology
Keywords: atomic mass, atomic weight, IUPAC, periodic table
The standard masses of 10 elements (including, from top, B, S, and Si) will now be given as a range of values.
Credit: images-of-elements.com/ Creative Commons
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The standard masses of 10 elements (including, from top, B, S, and Si) will now be given as a range of values.
Credit: images-of-elements.com/ Creative Commons

Breaking with a century-old tradition, the International Union of Pure & Applied Chemistry (IUPAC) is publishing a new table of standard atomic weights in which the values for 10 elements (H, Li, B, C, N, O, Si, S, Cl, Tl) will be given as mass ranges rather than single values. This historic change in format is being instituted to address the natural variation in a material’s isotope abundances, which depend on the sample’s physical, chemical, and nuclear history. For example, boron’s atomic mass until now has been given as 10.811 amu. The new tables will list the value as [10.806; 10.821] to reflect the element’s true atomic weight, which can vary over that range depending on the material’s source. “We’ve grown up thinking that standard atomic weights given in the periodic table are constants of nature,” says Tyler B. Coplen, an isotope specialist at the U.S. Geological Survey, in Reston, Va. Coplen, who is a coauthor of the IUPAC report highlighting these changes (Pure Appl. Chem., 10.1351/PAC-REP-10-09-14 ) and a companion paper in Chemistry International, adds that that fact holds true only for elements with just one stable isotope.

 
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