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    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    169Tm 168.934 219(5)1

    Thulium is a monoisotopic element and its atomic weight is determined solely by its isotope 169Tm. The Commission last revised the standard atomic weight of thulium in 2021 based on the latest Atomic Mass Evaluation by IUPAP.

    SOURCE  Atomic weights of the elements: Review 2000 by John R de Laeter et al. Pure Appl. Chem. 2003 (75) 683-800
    © IUPAC 2003


    Ar(Tm) = 168.934 219(5) since 2021

    The name derives from Thule, the earliest name for the northernmost part of the civilized world—Scandinavia. It was discovered in 1879 by the Swedish chemist Per Theodor Cleve in a sample of erbium mineral. Thulium was first isolated by the American chemist Charles James in 1911.