Question

Ratio of radii of third and second orbits of $H$ atom is:

• A. $3:2$
• B. $9:4$
• C. $4:1$
• D. $5:2$

Answer 1

The correct option is B $9:4$

Bohr's radius for $n^{th}$ orbit, $\left(r_n\right)=0.529\left[\frac{n^2}{Z}\right]\stackrel{\circ }{A}$
where $n=\text{Orbit number},Z=\text{Atomic number}$

Since, $r\propto n^2$
$\therefore \frac{r_2}{r_1}=\frac{3^2}{2^2}=\frac{9}{4}$