Dictionary![]() ![]() atom'ic mass'Chem. 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 the mass of one oxygen atom, but after 1961 based on 1/12 the mass of the carbon-12 atom. Abbr.: at. m. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.
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