Q 12-12-015NEETJEE MainMedium
The radius of the first Bohr orbit of hydrogen is $0.53$ Å. The radius of the second orbit of He$^+$ is
Answer: (D) $1.06$ Å
Concept. For a hydrogen-like ion $r_n = r_1\dfrac{n^2}{Z}$ with $r_1 = 0.53$ Å.
Step 1. Substitute. He$^+$: $Z = 2$, $n = 2$:
$$r = 0.53 \times \frac{4}{2} = 1.06\ \text{Å}$$
Answer. $1.06$ Å (Option D).
Solution by Sreeraj P, M.Sc Physics