Assume that a neutron breaks into a proton and an electron. The energy released during this process is
 (Mass of neutron = $1.6725\times {10}^{-27}kg$
 Mass of proton = $1.6725\times {10}^{-27}kg$ 
 Mass of electron = $9\times {10}^{-31}kg$)
 

A neutron breaks into a proton and electron.
Mass of neutron = $1.6725\times {10}^{-27}kg$
 Mass of proton = $1.6725\times {10}^{-27}kg$
 Mass of electron = $9\times {10}^{-31}kg$
$\Delta m=(m_p+m_e)-m_n$
Energy released$=-(9\times {10}^{-31}kg)C^{2}joule$

$=\frac{-9\times {10}^{-3}\times {(3\times {10}^8)}^2}{1.6\times {10}^{-13}} \mathrm{MeV}=-0.51 \mathrm{MeV}$