Explanation

(a) The radius of the Bohr atom model, which is valued only for hydrogen or other ionized atoms with a single electron is given by $r_{n} = \left( \frac{n^{2}}{m} \right)\left( \frac{h}{2\pi} \right)^{2}\left( \frac{4\pi\varepsilon_{0}}{Ze^{2}} \right)$
$\therefore$ For a given $Z = 82,r \propto n^{2}$
$\therefore$ If the radius of the first orbit is $R$, the radius of the third orbit is $n^{2}R\ i.e.,9R$