Nuclear and atomic masses, when expressed directly in kilograms, come out as extremely inconvenient numbers (of order 10−25 kg or smaller), so nuclear physics instead uses a purpose-built unit called the atomic mass unit, symbol u. One atomic mass unit is defined as exactly one-twelfth of the mass of a single atom of the most abundant naturally occurring isotope of carbon, 612C:
1 u=121×(mass of one 612C atom)=1.660×10−27 kg
Using this unit, key particle and atomic masses become simple, easily remembered numbers close to whole integers: the neutron's mass is 1.008665 u, the proton's mass is 1.007276 u, the hydrogen atom's mass is 1.007825 u, and by definition the mass of 612C is exactly 12 u.
It is important to keep in mind that a mass quoted "for" a nucleus (or, more usually, "for" an atom) in atomic mass units is nearly always the atomic mass - the nucleus plus its full complement of orbiting electrons - rather than the bare nuclear mass alone; to isolate the true nuclear mass, the combined mass of the atom's Z electrons must be subtracted from the tabulated atomic mass. Atomic masses are measured experimentally with an instrument called the Bainbridge mass spectrometer. …