Hydrogen atom has only one electron, so mutual repulsion between electrons is absent. However, in multielectron atoms mutual repulsion between the electrons is significant. How does this affect the energy of an electron in the orbitals of the same principal quantum number in multielectron atoms?
Unlike in the hydrogen atoms where the energy of an electron only depends upon the principal quantum number or the main shell, in case of multi-electron atoms the energy depends on another factor apart from n i.e. the value of l or orbital angular momentum or the sub-shell. And the energy of sub-shells for the same principal quantum number or the main shell increases in the order of s<p<d<f. And for the higher orbitals, this difference is pronounced to a larger extent which results in: 4s<3d or 6s<5d or 4s< 6p.
The main reason behind these differences is the mutual repulsion in multi-electron atoms which is absent in case of hydrogen. These repulsion besides the presence of interaction between electron and nucleus indicates that in the case of multi-electron atoms the stability comes for the occurrence of more powerful total attractive interactions over the repulsions.
