If $m, m_n$ and $m_p$ are the masses of ${ }_Z X^A$ nucleus, neutron and proton respectively

$m<(A-Z)_{m_n}+Z m_p$ $m=(A-Z) m_n+Z m_p$ $m=(A-Z) m_p+Z m_n$ $m>(A-Z) m_n+Z m_p$

$m<(A-Z)_{m_n}+Z m_p$

The mass of nucleus formed is always less than the sum of the masses of the constituent protons and neutrons i.e. $m<(A-Z) m_n+z m_p$.