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    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    203Tl 202.972 345(9)[0.2944, 0.2959]
    205Tl 204.974 428(9)[0.7041, 0.7056]

    In 1961, the Commission recommended Ar(Tl) = 204.37 based on the chemical determinations. In 1979, the Commission considered new mass-spectrometric measurement and recommended Ar(Tl) = 204.383(1). Recent studies have indicated substantial variation in n(205Tl)/n(203Tl) of natural materials, which led the Commission to adopt the interval notation for the standard atomic weight of thallium in 2009.

    SOURCES  Atomic weights of the elements: Review 2000 by John R de Laeter et al. Pure Appl. Chem. 2003 (75) 683-800
    Atomic weights of the elements 2009 by M.E. Wieser and T.B. Coplen. Pure Appl. Chem. 2011 (83) 359-396


    Ar(Tl) = [204.382, 204.385] since 2009

    The name derives from the Greek thallos for "green shoot" or "twig" because of the bright green line in its spectrum. Thallium was discovered by the English physicist and chemist William Crookes in 1861. Metallic thallium was first isolated by the French chemist Claude-Auguste Lamy in 1862.

    Natural variations of thallium isotopic composition

    Isotopic reference materials of thallium.