The number of $\alpha$ and ${\beta }^{-}$ particles emitted during the radioactive decay chain starting from  $R{a}_{88}^{226}$  and ending at  $P{b}_{82}^{206}$  is

By the emission of one $\alpha$ particle, atomic number decreases by 2 and mass number by 4. By the emission of one ${\beta }^{-}$ particle atomic number increases by 1 and mass number remain same.