mass atomic mass The total mass of an atom, equal to the mass of the atom's constituent protons, neutrons and electrons minus the nuclear and electron binding energies. Atomic mass is expressed in atomic mass units (amu), where 1 amu is equal to one twelfth the mass of an atom of carbon-12, which is equal to 1.660539040 � 10?24 gram. The amu is also called the Dalton (Da).