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    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    168Yb 167.933 889(8)0.001 26(1)
    170Yb 169.934 767 25(7)0.030 23(2)
    171Yb 170.936 331 52(9)0.142 16(7)
    172Yb 171.936 386 66(9)0.217 54(10)
    173Yb 172.938 216 22(8)0.160 98(9)
    174Yb 173.938 867 55(8)0.318 96(26)
    176Yb 175.942 5747(1)0.128 87(30)

    The atomic weight of ytterbium has been taken as 173.04 since 1934. This chemically determined value was reconfirmed by the Commission in 1961. However, from that time, Ar(Yb) was no longer based on chemical but on mass-spectrometric data. In light of new mass-spectrometric determinations, the Commission recommended Ar(Yb) = 173.054(5) in 2007. In 2015, the standard atomic weight of ytterbium was revised to 173.045(10). Ytterbium, which was first obtained in a pure state just some 50 years ago, only two calibrated measurements of its isotopic composition have been ever made. Both of these measurements differ from one another and ytterbium exemplifies the situation that is also true for many other elements: well-documented isotope ratio measurements are still needed.
    The "g" annotation is derived from anomalous Yb in samples from the Oklo uranium deposit in Gabon, south-west Africa.


    Ar(Yb) = 173.045(10) since 2015

    The name derives from the Swedish village of Ytterby where the mineral ytterbite (the source of ytterbium) was originally found. It was discovered by the Swiss chemist Jean-Charles Galissard de Marignac in 1878 in erbium nitrate from gadolinite (ytterbite renamed).