Before applying these ideas to the nucleus we must first introduce the term unified mass unit, u. The mass of the nucleus is of the order of 10 ~27 kg. Clearly, a kilogram is too large a unit for mass in this context and it is necessary to define a more convenient unit. This is the unified mass unit u and is defined on the basis that the mass of the carbon nuclide C-12 is exactly 12 u [the unified mass unit supersedes the previous unit of atomic mass unit based on the mass of oxygen 0-16 being 16 amu]. From this definition:

lu = 1.6604 x 10-27 kg

Applying Einstein’s equation E = m C2 with m in kg and C in m/s (= 3 x 10s), giving E in joules, then:

lu = 1.6604 x 10’27 x 9 x 1016 = 1.4944 x 10“10 joules

As previously, the kilogram was found to be inade­quate as a unit for mass, the joule also is too large a unit for energy. A more convenient energy unit is the electron volt, eV, and is the energy acquired by an electron in accelerating through a potential difference of 1 volt. Using these new units and recalling that the charge on an electron is 1.6022 x 10“19 coulomb, we have:

1 eV = 1.6022 x 10“19 joules

Therefore 1 joule = 6.242 x 1012 MeV Since lu = 1.6604 x 10“27 kg

lu ■ 931 MeV

Also:

Mass of the neutron = 1.008665 u

Mass of the proton = 1.007277 u

Mass of the electron = 0.000548 u