Atomic mass

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.
Historical Examples

The best evidence that these particles are of atomic mass is furnished by their deflection in electric and magnetic fields.
A Brief Account of Radio-activity Francis Preston Venable

noun (chem)
the mass of an isotope of an element in atomic mass units
short for relative atomic mass See atomic weight

atomic mass n.
The mass of an atom, usually expressed in atomic mass units.
atomic mass
The mass of a given atom or molecule, expressed in atomic mass units. Compare atomic weight. See also mass number.


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