Question

The radionuclide ${}_{90}^{234}Th$ undergoes two successive $\beta$-decays followed by one $\alpha$-decay. The atomic number and the mass number respectively of the resulting radionuclide are

• A. $94$ and $230$
• B. $90$ and $230$
• C. $92$ and $230$
• D. $92$ and $234$

Answer 1

The correct option is B $90$ and $230$

For first Beta decay,
${}_{80}^{234}Th{\to }_{81}^{234}Th+e^{-}+\overline{v}$

For second beta decay,

${}_{81}^{234}Th{\to }_{92}^{234}Th+e^{-}+\overline{v}$

For $\alpha$-decay,

${}_{92}^{234}{Th{\to }_{90}^{234}Th+}_2^{4}He$

Therefore, Resulting mass-number=230

Atomic-number=90