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    Lutetium

    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    175Lu 174.940 78(2)0.974 01(13)
    176Lu 175.942 69(2)0.025 99(13)

    In 1961, the Commission recommended Ar(Lu) = 174.97 based on new mass-spectrometric determinations. The Commission also noted the close agreement with the chemical determinations Ar(Lu). In 1977, it took note of a new isotope-abundance determination, which was deemed more accurate but, like its predecessors, was not calibrated. Nevertheless, as a result, the more precise Ar(Lu) = 174.967(3) was recommended. Moreover, in 1981 the Commission was able to reduce the quoted uncertainty from 0.003 to 0.001. This change was justified because another abundance determination by agreed very well with earlier measurements. The Commission last revised the atomic weight of lutetium in 2007.

    The minor isotope, 176Lu, is radioactive with a half-life of 3.57(14)×1010 a. In consequence, Ar(Lu) will change comparably with the current uncertainty (0.0001) in only about 5×108 a. At Oklo, the n(176Lu)/n(175Lu) ratio has been used as a sensitive measure of the average equilibrium temperature of the neutrons at the time of the nuclear reactions. The occurrence at this site of almost pure (99.7 %) isotope 175Lu justifies the annotation "g". The 176Lu–176Hf decay system is used as a geochronometer.

    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

    Lutetium
    Ar(Lu) = 174.9668(1) since 2007

    The name derives from Lutetia, the ancient name for the city of Paris. The discovery of lutetium is credited to the French chemist Georges Urbain in 1907 although it had been separated earlier and independently by the Austrian chemist Carl Auer (Baron von Welsbach) from an ytterbium sample.
    Von Welsbach had named the element cassiopeium after the constellation Cassiopeia. However, because Urbain published his results before Auer, his name for the element was adopted by IUPAC in 1949.