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    Selenium

    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    74Se 73.922 4759(1)0.0086(3)
    76Se 75.919 2137(2)0.0923(7)
    77Se 76.919 9142(5)0.0760(7)
    78Se 77.917 309(2)0.2369(22)
    80Se 79.916 522(8)0.4980(36)
    82Se 81.916 700(9)0.0882(15)

    In 1934, the Commission recommended Ar(Se) = 78.96 based on the chemical determinations. This value was reaffirmed in 1969 when Ar(Se) = 78.96(3) was recommended. Both the atomic weight and the uncertainty values have since remained unchanged until 2013 when new mass-spectrometric measurements enabled to recommend a more precise value Ar(Se) = 78.971(8).

    82Se is radioactive with an enormously long half-life of approximately 1020 a and is considered to be a stable isotope.

    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

    Selenium
    Ar(Se) = 78.971(8) since 2013

    The name derives from the Greek Selene, who was the Greek goddess of the Moon because the element is chemically found with tellurium (Tellus was the Roman goddess of the Earth). Selenium was discovered by the Swedish chemist Jöns Jacob Berzelius in 1817, while trying to isolate tellurium in an impure sample.

    Isotopic reference materials of selenium.