Two electrons are revolving around a nucleus at distances $r$ and $4r$. The ratio of their time periods is

An electron in a hydrogen atom in its ground state absorbs $2$ times as much energy as the minimum required for it to escape from the atom. The kinetic energy of the emitted electron is ............ $eV$.

If the electron of a hydrogen atom is in the third orbit,it represents its ...... excited state.

Calculate the radius of the fourth orbit of the $B^{4+}$ ion. (in $pm$)

Explain the photoelectric effect.

The ratio between the radii of $He^{+}$ ion and $H$ atom is: