Atomic weight

Not-so-constant weights

Print edition : July 12, 2013

STANDARD atomic weights for chemical elements, we were told from school days, are constants of nature, along with the speed of light and the attraction of gravity. The International Union of Pure and Applied Chemistry (IUPAC) Commission on Isotopic Abundances and Atomic Weights has published a new table that expresses the standard atomic weights of magnesium and bromine as intervals, rather than as single specific values. In addition, improved standard atomic weights have been determined for germanium, indium, and mercury. This new table is the result of cooperative research supported by the United States Geological Survey, the IUPAC, and other contributing Commission members and institutions.

Modern analytical techniques can measure the atomic weights of many elements with such precision that small variations in an element’s atomic weight serve as markers for certain physical, chemical and biological processes.

Atoms of the same element that have different masses are called “isotopes”. The atomic weight of an element depends upon how many stable isotopes it has and the relative amounts of each stable isotope present in a sample containing the element.

Elements with only one stable isotope each do not exhibit variations in their atomic weights. For example, the standard atomic weights for fluorine, aluminium, sodium and gold are constant. Their values are known to better than six decimal places. Variations in atomic weight occur when an element has two or more naturally occurring stable isotopes in varying abundances in a given sample.

The standard atomic weights of magnesium and bromine will now be expressed as intervals to convey this variation in atomic weight more accurately. For example, bromine commonly is considered to have a standard atomic weight of 79.904. However, its actual atomic weight can be anywhere between 79.901 and 79.907, depending on where the element is found.

The IUPAC, which oversees the periodic evaluation and dissemination of atomic weight values, has previously adjusted the standard atomic weights of the elements hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulphur, chlorine and thallium as intervals to reflect variations in their atomic weights. The new table of updated atomic weights has been published in the journal Pure and Applied Chemistry.

This new precise determination of atomic weights does have practical applications as well. For example, precise measurements of the abundances of isotopes of carbon can be used to determine the purity and source of food products such as vanilla and honey. Isotopic measurements of nitrogen, chlorine and other elements are used for tracing pollutants in streams and groundwater. In investigations of sports doping, performance-enhancing testosterone can be identified in the human body because the atomic weight of carbon in natural human testosterone is different from that in pharmaceutical testosterone.

R. Ramachandran

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