The key feature of Bohr's theory of spectrum of hydrogen atom is the quantization of angular momentum when an electron is revolving around a proton. We will extend this to a general rotational motion to find quantized rotational energy of a diatomic molecule assuming it to be rigid. The rule to be applied is Bohr's quantization condition.

In a CO molecule, the distance between C (mass $=12$ a.m.u) and O (mass $=16$ a.m.u.), where 1 a.m.u. $=\frac{5}{3} \times 10^{-27} \mathrm{~kg}$, is close to