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    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    159Tb 158.925 354(7)1

    Terbium is a monoisotopic element and its atomic weight is determined solely by its isotope 159Tb. The Commission last revised the standard atomic weight of terbium 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(Tb) = 158.925 354(7) since 2021

    The name derives from the village of Ytterby in Sweden, where the mineral ytterbite (the source of terbium) was first found. Terbium was discovered by the Swedish surgeon and chemist Carl-Gustav Mosander in 1843 in an yttrium salt, which he resolved into three elements. He called one yttrium, a rose-colored salt he called terbium, and a deep-yellow peroxide he called erbium. In 1862, the Swiss chemist Marc Delafontaine reexamined yttrium and found the yellow peroxide. Because the name erbium had now been assigned to the rose-colored oxide, he reintroduced the name terbium for the yellow peroxide. Thus the original names given to erbium and terbium samples are now switched.