The International Union of Pure and Applied Chemistry (Iupac) has revised the standard atomic weight of ytterbium from 173.054 to 173.045.

The change is based on a recent measurement of ytterbium’s isotopic abundances, which differs from previous observations. There are several naturally occurring isotopes of the element from ytterbium-168 to ytterbium-176. Its standard atomic weight was first recorded as 173.04 in 1934, and confirmed by Iupac in 1961 using chemical determination. This was revised to 173.054 in 2007 when new methods based on mass spectrometry were used to make the calculations.

At a recent discussion, Iupac’s commission on isotopic abundances and atomic weights (CIAAW) pointed out that there have only been two calibrated measurements of ytterbium’s isotopic composition since the element was first purified 50 years ago, both of which returned different values. CIAAW noted the need for more measurements to be taken, as the lack of data on isotopic abundance is an issue affecting a number of other elements.