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    Barium

    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    130Ba 129.906 32(2)0.0011(1)
    132Ba 131.905 061(7)0.0010(1)
    134Ba 133.904 508(2)0.0242(15)
    135Ba 134.905 688(2)0.0659(10)
    136Ba 135.904 576(2)0.0785(24)
    137Ba 136.905 827(2)0.1123(23)
    138Ba 137.905 247(2)0.7170(29)

    In 1961, the Commission recommended a value of Ar(Ba) = 137.34 based both on the chemical data and on mass-spectrometric abundance measurements. In 1975, the Commission reviewed the above data as well as new mass-spectrometric measurements. All of these measurements fell within a very narrow range, and recommended Ar(Ba) = 137.33(1). This value was revised in 1985 to 137.327(7) as a result of new guidelines for evaluating the uncertainty of atomic weights.

    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

    Barium
    Ar(Ba) = 137.327(7) since 1985

    The name is derived from the Greek barys for "heavy" because it was found in the mineral heavy spar (BaSO4). It was discovered by the Swedish pharmacist and chemist Carl Wilhelm Scheele in 1774 and first isolated by the British chemist and physicist Humphry Davy in 1808.