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    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    184Os 183.952 493(6)0.0002(2)
    186Os 185.953 838(5)0.0159(64)
    187Os 186.955 750(5)0.0196(17)
    188Os 187.955 837(5)0.1324(27)
    189Os 188.958 146(5)0.1615(23)
    190Os 189.958 446(5)0.2626(20)
    192Os 191.961 48(2)0.4078(32)

    In 1969, the Commission recommended Ar(Os) = 190.2(l). At this time, osmium had the largest uncertainty among all standard atomic weights, equalled with lead (for which, however, the atomic weight is affected by natural variability, not applicable to osmium). In 1991, the Commission considered new isotope measurements yielding an atomic weight having a significantly improved precision, and recommended Ar(Os) = 190.23(3).

    One of the minor isotopes, 186Os, is radioactive with a very long half-life of 2.0×1015 a. It undergoes α-decay into stable 182W but does not affect Ar(Os) even over geologic time. 187Os is the stable product of β– active 187Re decay. As a result, osmium occurs with anomalous atomic weight as a trace element in Re-bearing rocks. The "g" annotation is thereby justified.

    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(Os) = 190.23(3) since 1991

    The name derives from the Greek osme for "smell" because of the sharp odor of its volatile oxide. Both osmium and iridium were discovered simultaneously in a crude platinum ore by the English chemist Smithson Tennant in 1803.

    Isotopic reference materials of osmium.