The binding energy of a H -atom, considering an electron moving around a fixed nuclei (proton), is

$$B=-\frac{m e^4}{8 n^2 \varepsilon_0^2 h^2}(m=\text { electron mass })$$

If one decides to work in a frame of reference where the electron is at rest, the proton would be moving around it. By similar arguments, the binding energy would be

$$B=-\frac{M e^4}{8 n^2 \varepsilon_0^2 h^2}(M=\text { proton mass })$$

This last expression is not correct, because

The simple Bohr model cannot be directly applied to calculate the energy levels of an atom with many electrons. This is because

For the ground state, the electron in the H -atom has an angular momentum $=h$, according to the simple Bohr model. Angular momentum is a vector and hence there will be infinitely many orbits with the vector pointing in all possible directions. In actuality, this is not true,

$\mathrm{O}_2$ molecule consists of two oxygen atoms. In the molecule, nuclear force between the nuclei of the two atoms

Two H atoms in the ground state collide inelastically. The maximum amount by which their combined kinetic energy is reduced is