An Etymological Dictionary of Astronomy and Astrophysics

The mass of a single atom, when the atom is at rest at its lowest energy level (→ ground state). Because a → chemical element may exist as various → isotopes, possessing different numbers of neutrons in their atomic nuclei, atomic mass is calculated for each isotope separately. Atomic mass is most often expressed in unified → atomic mass units, where one unified atomic mass unit is defined as one-twelfth the mass of a single atom of the carbon-12 isotope.

See also: → atomic; → mass.