The masses of parent and daughter elements nuclide are ${\mathrm{m}}_1\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{ }{\mathrm{m}}_2$ respectively.

Mass of parent element = ${\mathrm{m}}_1+\mathrm{ }\mathrm{Z}{\mathrm{m}}_{\mathrm{e}}$ (where Z is atomic number of parent element)

Mass of daughter element - ${\mathrm{m}}_2+\mathrm{ }\left(\mathrm{Z}+1\right){\mathrm{m}}_{\mathrm{e}}\mathrm{ }$ (As atomic number of element increases by one due to $\mathrm{\beta }\mathrm{ }–$ emission)

Mass decay = mass of parent atom - one $\mathrm{\beta }$ loss - [mass of daughter atom along with one electron]

${=\mathrm{m}}_1+\mathrm{Z}{\mathrm{m}}_{\mathrm{e}}-{\mathrm{m}}_{\mathrm{e}}-[{\mathrm{m}}_2+\mathrm{ }\left(\mathrm{Z}+1\right){\mathrm{m}}_{\mathrm{e}}]$

$=[{\mathrm{m}}_1-{\mathrm{m}}_2-2{\mathrm{m}}_{\mathrm{e}}]$

Energy liberated $=\left[{\mathrm{m}}_1-{\mathrm{m}}_2-\mathrm{ }2{\mathrm{m}}_{\mathrm{e}}\right]{\mathrm{c}}^2$