It is observed that the mass of a nucleus is slightly less than the sum of the masses of constituent nucleons. Suppose a nucleus consists of 'Z' protons and 'N' neutrons. Mass of a proton, a neutron and the resulting nucleus are respectively `m_p, m_n` and M then mass defect of the nucleus is given by

`Deltam=Zm_p+Nm_n-M`

If A is the mass number of the nucleus

`Deltam = [Zm_p + (A - Z)m_n - M]`

In terms of atomic masses we may also write mass defect as

`Deltam=[Zm(text()_1^1H)+Nm_n-m(text()_Z^AX)]`

Where `m(text()_1^1H)` = mass of one hydrogen atom.

`m(text()_Z^AX)` = mass of atom having atomic no. Z and mass no. A.