The radius of electron's second stationary orbit in Bohr's atom is $$R$$. The radius of 3$$^{rd}$$ orbit will be

Given that the radius of the electron's second stationary orbit in Bohr's atom is R, we note that in Bohr's model the radius of the n-th orbit is proportional to n^2: $$ r_n = r_0 \cdot n^2 $$ where $$r_0$$ is the radius of the first orbit.

For n = 2, this gives $$ r_2 = r_0 \times 4 = R $$, and thus $$ r_0 = \frac{R}{4} $$.

Substituting into the expression for the third orbit (n = 3) yields $$ r_3 = r_0 \times 9 = \frac{R}{4} \times 9 = \frac{9R}{4} = 2.25R $$. Therefore, the radius of the third orbit is 2.25R.