| 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. Abbreviation: at. m. |

| atomic mass n. The mass of an atom, usually expressed in atomic mass units. |
atomic mass n.
The mass of an atom, usually expressed in atomic mass units.
atomic mass
the quantity of matter contained in an atom of an element. It is expressed as a multiple of one-twelfth the mass of the carbon-12 atom, 1.992646481023gram, which is assigned an atomic mass of 12 units. In this scale 1 atomic mass unit (amu) corresponds to 1.660538731024gram.
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