Revised Atomic Weights

Mass spectrometry—a technology that is constantly being made more accurate and reliable—is used every two years to revise the standard atomic weights of select elements. This year, the International Union of Pure & Applied Chemistry (IUPAC), which oversees these revisions, has released revised atomic weights for 19 elements.

But a planned redefinition of the kilogram, the standard unit of mass, could take the reassessments of atomic weights to a new level of accuracy.

This year’s element revisions include changes in atomic weights for cadmium, molybdenum, selenium, and thorium, on the basis of recent determinations of terrestrial abundances of each element’s multiple stable isotopes. Revisions of the other 15 elements reflect updated assessments of their single stable isotopes.

“High-precision measurements of atomic weights play an important role in science,” comments Juris Meija, secretary of IUPAC’s Commission on Isotopic Abundances & Atomic Weights. Although the weight changes don’t affect the daily work of most scientists, they can have wide-ranging effects. In the 1980s, for example, measurements of the atomic weight of silver helped settle the value of the Faraday constant, Meija explains. “Nowadays, the atomic weight of silicon is an important variable in setting the high-precision values for the Planck constant and for the Avogadro constant.”

Both constants are being used to redefine the kilogram, Meija says, which is the only standard unit of measure in the metric system still based on a physical artifact—a platinum-iridium cylinder in Severn, France. Redefining a kilogram in terms of universal constants would allow the standard to be reproduced accurately without having to physically measure an artifact.

A fixed value for Avogadro’s number and a kilogram defined by constants would, in effect, shift any uncertainty onto the mass of ¹²C, which is taken to be an exact value. Subsequently, any revisions of the mass of ¹²C would require a slight recalibration of mass spectrometers.

The Committee on Data for Science & Technology, a sister organization to IUPAC, will make the final determination on redefining the kilogram. IUPAC’s new atomic weight revisions are, in some ways, “creating the foundation for this redefinition,” says Carl J. Williams of NIST’s Physical Measurement Laboratory.