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    Cerium

    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    136Ce 135.907 129(3)0.001 86(2)
    138Ce 137.905 99(7)0.002 51(2)
    140Ce 139.905 44(2)0.884 49(51)
    142Ce 141.909 25(2)0.111 14(51)

    In 1961, the Commission recommended Ar(Ce) = 140.12 based on the average value of the mass-spectrometric measurements, which were in good agreement with earlier chemical determinations. The atomic weight and uncertainty of cerium were changed to their current values in the 1995 report of the Commission, based on new isotope-abundance data.

    138Ce and 140Ce are the decay products of long-lived minor isotopes 138La and 144Nd, respectively. They have a negligible effect on Ar(Ce) in normal sources, but add justification to the "g" annotation, which also refers to the Oklo occurrence in 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

    Cerium
    Ar(Ce) = 138.905 47(7) since 1995

    The name derives from the planetoid Ceres, which was discovered by the Italian astronomer Giuseppe Piazzi in 1801 and named for Ceres, the Roman goddess of agriculture and harvest. Two years later, the element cerium was discovered by the German chemist Martin-Heinrich Klaproth, who called it ochroeite earth because of its yellow colour.
    Cerium was independently discovered at the same time by the Swedish chemist Jöns Jacob Berzelius and the Swedish mineralogist Wilhelm von Hisinger, who called it ceria. It was first isolated in 1875 by the American mineralogist and chemist William Frances Hillebrand and the American chemist Thomas H. Norton.