When an alpha particle $\left({}_2^4\text{He}\right)$  is emitted, the mass number and the atomic number of the daughter nucleus decreases by four and two respectively. When a beta particle (β^–) is emitted, the atomic number of the daughter nucleus increases by one but the mass number remains the same.

$\therefore {}_n^{m}X\stackrel{\alpha }{⟶}{}_{n-2}^{m-4}Y\stackrel{2{\beta }^{-}}{⟶}{}_n^{m-4}X$