Ytterbium is a bit lighter than previously thought: Its standard atomic weight is now 173.045, down from 173.054, as approved by the International Union of Pure & Applied Chemistry (IUPAC) in August.
The IUPAC Commission on Isotopic Abundances & Atomic Weights evaluates atomic weights every two years. Which elements it tackles depends on what new data are available. This year, only ytterbium made the docket, based on mass spectrometric analysis carried out by China’s National Institute of Metrology (J. Anal. At. Spectrom. 2015, DOI: 10.1039/c5ja00054h).
On average, the atomic weights of individual elements get updated about every 14 years, says the commission’s chair, Juris Meija of the National Research Council of Canada. Some last longer than others—lead’s was last set in 1969, for example—not necessarily because the numbers are solid but because the experiments require expensive, isotopically enriched samples, Meija says.
In ytterbium’s case, its standard atomic weight is calculated as a weighted average of seven isotopes, based on quantifying their natural abundances on Earth. IUPAC revised the weight to 173.04 in 1934 based on chemical analysis that was later confirmed by mass spectrometry. The union then adjusted the value to 173.054 in 2007 on the basis of additional mass spectrometric work that was combined with the latest results for this year’s update, Meija says.