The ratio of the radius of the third orbit to that of the second orbit in a hydrogen atom is

9 : 4

The correct answer is Option (2) → 9 : 4

For hydrogen atom, the radius of nth orbit is given by:

$r_n = n^2 a_0$

Hence, the ratio of the radius of the third orbit to that of the second orbit is:

$\frac{r_3}{r_2} = \frac{3^2 a_0}{2^2 a_0} = \frac{9}{4}$

∴ The required ratio is $\frac{9}{4}$ or 9 : 4