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    Erbium

    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    162Er 161.928 787(6)0.001 39(5)
    164Er 163.929 207(5)0.016 01(3)
    166Er 165.930 299(8)0.335 03(36)
    167Er 166.932 054(8)0.228 69(9)
    168Er 167.932 376(8)0.269 78(18)
    170Er 169.935 47(1)0.149 10(36)

    In 1969, the Commission assessed Ar(Er) = 167.26(3). The atomic weight and uncertainty of erbium were changed to their current values in 1999 as a result of new mass-spectrometric measurements. The "g" notation arises from the presence of naturally occurring fission products found in fossil reactors at Gabon, south-west Africa.

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

    CIAAW

    Erbium
    Ar(Er) = 167.259(3) since 1999

    The name derives from the Swedish town of Ytterby, where the ore gadolinite (in which it was found) was first mined. Erbium was discovered by the Swedish surgeon and chemist Carl-Gustav Mosander in 1843 in a yttrium sample. He separated the yttrium into yttrium, a rose-coloured salt he called terbium and a deep-yellow peroxide that he called erbium.