- the mass of an isotope of an element measured in units formerly based on the mass of one hydrogen atom taken as a unit or on 1/16 (0.0625) the mass of one oxygen atom, but after 1961 based on 1/12 (0.0833) the mass of the carbon-12 atom. Abbreviation: at. m.
Origin of atomic mass
First recorded in 1895–1900
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- The mass of an atom, usually expressed in atomic mass units.
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