Atomic weights: not so constant after all

The IUPAC Commission on Isotopic Abundances and Atomic Weights has changed the way certain atomic weights are reported [1–3]. In particular, the fixed ‘standard atomic weight’ has been replaced by an atomic weight ‘interval’ for ten predominantly light elements (H, Li, B, C, N, O, Si, S, Cl and Tl), reflecting the variability of their isotopic composition. Although the respective span (the ‘range’) may not be large, the precise atomic weight of an element in a particular batch or reservoir has to be estimated from other material properties, or has to measured with higher precision, depending on experimental requirements. In the future, the atomic weight intervals are expected to increase with new findings, such as the discovery of new materials or isotopically exotic sources of materials, rather than becoming more tightly constrained by new measurements