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    Atomic Masses

    Atomic masses of isotopes are evaluated regularly by the Atomic Mass Data collaboration under the auspices of the IUPAP. The table below contains data from the Atomic Mass Evaluation reports, including the most recent 2020 update. All atomic masses of nuclides, ma, are expressed in daltons (symbol Da), which is 1/12 mass of a single carbon-12 atom (unbound, neutral, in its electronic and nuclear ground state, and at rest).

    Please enter the symbol of the element, isotope, or the year in order to retrieve the relevant atomic masses. Add a '=' sign at the end of the query for exact match.

    All names and symbols of the elements are given in their present form even though many changes have occured in this regard over the last decades.

    SOURCE   Data compiled programmatically by Juris Meija from the AME data files spanning time period of 1983-2020.

    The CIAAW uses nuclide masses as reported in the Atomic Mass Evaluation reports to calculate atomic weights. However, the uncertainty of the nuclide masses is not taken as reported by the AME. Rather, all uncertainty estimates are expanded by a factor of k = 6 in order to conform to the conservative reporting practices of the CIAAW.

    The entire datafile (CSV) is available for download here.
    Last updated: 17 March 2021