Chemistry’s standardisation body introduces atomic weight interval for argon and updates weights of another 13 elements

Argon’s days of having a fixed atomic weight are gone. For the first time since its discovery in 1894, the noble gas’s weight will be defined by an interval, reflecting how its isotope composition varies depending on the source. The International Union of Pure and Applied Chemistry (Iupac) also updated the atomic weights of another 13 elements.

Argon has three stable isotopes, but their ratio depends on where the gas comes from. On Earth, radioactive potassium-40 decays to argon-40 as the most common isotope. In stars, nucleosynthesis mostly creates the lighter argon-36 and argon-38. But even in terrestrial deposits, ‘isotopic abundance of argon-40 varies from around 94% to 100% depending on where it is from’, explains Juris Meija, who heads Iupac’s commission on isotopic abundances and atomic weights. In terrestrial samples the ratio of argon-36 to 38 to 40 is usually around 0.3:0.1:99.6, while in the sun it’s closer to 84:15:1. Read more.