The ratio of the area covered by the second orbital to the first orbital is

The radii of two of the first four Bohr’s orbits of the hydrogen atom are in the ratio $1 : 4$. The energy difference between them may be

The binding energy of an $e^-$ in the ground state of a hydrogen atom is $13.6 \ eV$. The energy required to remove an electron from the ground state of a $He^+$ ion is:

The ratio of the time period of an electron in the first and third orbit of an $H$-atom would be:

The ground and first excited state energies are $E_1$ and $E_2$ respectively. Which pair of species has the same energy? [Note that energy is indicated in the bracket].

Which were the major problems before the scientists after the discovery of sub-atomic particles?