December 2, 2013

New Atomic Weights for 19 Elements

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Chemistry students may have to update their periodic tables, as the International Union of Pure and Applied Chemistry revised the standard atomic weights of 19 elements this past September.

The new atomic weights were determined using more precise instruments and calculations.

Among the 19 revised elements are some commonly known elements, including fluorine, aluminum, phosphorus and gold, according to the Commission on Isotopic Abundances and Atomic Weights website. The other elements are: arsenic, beryllium, cadmium, cesium, cobalt, holmium, manganese, molybdenum, niobium, praseodymium, scandium, selenium, thorium, thulium and yttrium.

Atomic weights are measured in atomic mass units, the equivalent of one-twelfth the mass of a carbon atom with six neutrons. Researchers determine standard atomic weights by averaging the relative weight of an element’s isotopes. Isotopes of the same element have the same number of protons but a different number of neutrons, which affects how heavy an atom is. A single atom can have several isotopes, which can result in a wide variation of atomic weights.

The recent changes made to the atomic weights are minor — usually a difference of one-hundredth or less of an amu. For instance, the atomic weight of cobalt was changed from 58.933195 to 58.933194 amu, according to the CIAAW website.

“One reason for changes in the quoted values of atomic weights is that the masses of individual atoms can be measured with higher and higher precision as instrumentation improves,” Prof. David Zax, Department of Chemistry and Chemical Biology, said. “For other elements, and in particular where the quoted mass values are much less precise, the source of uncertainty — which we hope we can reduce with time — is almost always that there are two or more isotopes of the element.”

According to Zax, it is “notoriously difficult” to precisely determine the relative prevalence of each isotope for an element, especially for heavier elements. Heavy elements can be made several different ways, so researchers cannot assume that isotopic distribution in a man-made sample is representative of the average distribution.

Research that relies on the precise atomic weight of elements may be affected by the new changes. Otherwise, the changes do not have a deep impact, even on research in the field of chemistry.

“As best I can tell in a quick scan, the new values are always within the boundaries of what the old values were when one incorporates the uncertainty with which the old values were quoted,” Zax said. “As a result, the implications of these changes for anything anyone cares about would seem to be negligible.”

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