Question

An element X decays, first by positron emission and then two $\alpha$-particles are emitted in successive radioactive decay. If the product nucleus has a mass number $229$ and atomic number $89$, the mass number of atomic number of element X are

• A. $237,93$
• B. $237,94$
• C. $221,84$
• D. $237,92$

Answer 1

The correct option is B $237,94$

$X\to e^{+}+2\alpha +Y$
$\alpha =H{e}^{2+}$
Mass of $\alpha =4$
Mass no. of $Y=229$
By conservation of mass:
Mass no. of $X=My+8=237$
Conservation of charge:
$\alpha =2n+2p$
No. of protons in $Y=89$
No. of protons in $X=89+4+1$(positron) $=94$
Hence, atomic no. of $X=94$