Hydrogen atom has only one electron, so the mutual repulsion between the electrons is non-existent. However, in multielectron atoms the interaction between electrons is important. This is because, in the hydrogen atom, electron energy is determined by n value whereas in the multielectron atom, it is determined by the value of n+l . Thus, for a given principal quantum, the number of electrons in the s, p, d and f-orbtials have different energy.