If m, ${\mathrm{m}}_n\text{ and }{\mathrm{m}}_p$ are the masses of ${\mathrm{z}}^{{\mathrm{X}}^{\mathrm{A}}}$ nucleus, neutron and proton respectively

The mass of nucleus formed is always less than the sum of the masses of the constituent protons and neutrons 
${\text{ i.e. m<(A−Z)m}}_{\mathrm{n}}+{\mathrm{zm}}_{\mathrm{p}}$