When the radioactive isotope _ 88 R a^ 226 decays in a series by...

Due to emission of $3 \alpha$ particles, the mass number is reduced by 12 while atomic number is decreased by 6 . Due to emission of $\beta$ particle, the atomic number is increased by one while there is no change in mass number. Now, resulting mass will be decreased by 12 and atomic number is decreased by 5. So, isotope $X$ is ${ }_{83} X^{214}$.