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    Iron

    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    54Fe 53.939 609(3)0.058 45(105)
    56Fe 55.934 936(3)0.917 54(106)
    57Fe 56.935 393(3)0.021 19(29)
    58Fe 57.933 274(3)0.002 82(12)

    In 1961, the Commission recommended Ar(Fe) = 55.847(3) based on the average value of two reported mass-spectrometric determinations. In 1993, the Commission changed the recommended value for the standard atomic weight to Ar(Fe) = 55.845(2) based on calibrated mass-spectrometric measurements carried out on a metallic iron sample of high purity.

    Several studies have indicated natural isotope fractionation in iron-containing materials. The magnitude of the uncertainty assigned to the atomic-weight value was based mainly on the reported variations of Fe isotopic composition; however, subsequent studies now indicate somewhat different ranges. According to the compilation by the Commission, reported δ56Fe values range from −2.9 ‰ with Ar(Fe) = 55.8448 in human blood to +1.36 ‰ with Ar(Fe) = 55.8453 in part of a banded iron formation. Here δ56Fe refers to n(56Fe)/n(54Fe) relative to the reference material IRMM-014.

    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

    Iron
    Ar(Fe) = 55.845(2) since 1993

    The name derives from the Anglo-Saxon iron of unknown origin. The element has been known from prehistoric times. The symbol Fe is derived from the Latin ferrum for "firmness". It is of interest to note that 56Fe requires more energy to be formed than any other nuclide. It is, therefore, the ultimate endproduct of stellar nuclear fusion.

    Isotopic reference materials of iron.