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    IsotopeAtomic mass (Da)Isotopic abundance (amount fraction)
    133Cs 132.905 451 96(6)1

    Caesium is a monoisotopic element and its atomic weight is determined solely by its isotope 133Cs. The Commission last revised the standard atomic weight of caesium in 2013 based on the latest Atomic Mass Evaluation by IUPAP.

    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(Cs) = 132.905 451 96(6) since 2013

    The name derives from the Latin caesius for "sky blue", which was the colour of the caesium line in the spectroscope. Caesium was discovered by the German chemist Robert Wilhelm Bunsen and the German physicist Gustav Robert Kirchhoff in 1860. It was first isolated by the German chemist Carl Setterberg in 1882.