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Q.
A uranium nucleus ${ }_{92} U ^{238}$ emits an $\alpha$ - particle and $\beta$-particle in succession. The atomic number and mass number of the final nucleus will be
Nuclei
Solution:
When a radioactive nucleus emits an $\alpha$ -particle (a helium nucleus ${ }_{2} He ^{4}$ ), its mass number decreases by $4$ while the atomic number decreases by $2$. When a radioactive nucleus emits a $\beta$ -particle (an electron, $e^{-}$ ) its mass number remains unchanged while the atomic number increases by $1$.
$\therefore {}_{92} U ^{238} \xrightarrow{\alpha}{ }_{90} Th ^{234} \xrightarrow{\beta}{ }_{91} Pa ^{234}$