Atomic mass
The atomic mass ("m") is the mass of an atom. Its unit is the unified atomic mass units ("abbr." u) where 1 unified atomic mass unit is defined as of the mass of a single carbon-12 atom, at rest. For atoms, the protons and neutrons of the nucleus account for nearly all of the total mass, and the atomic mass measured in u has nearly the same value as the mass number.
Semi-empirical mass formula
The mass of an atomic nucleus is given by
where formula_2 and formula_3 are the rest mass of a proton and a neutron, respectively, and formula_4 is the binding energy of the nucleus. The semi-empirical mass formula states that the binding energy will take the following form: