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 prinicipal quantum number in multielectron atoms?

Explanation

In case of hydrogen atom, the energies of the electron in different orbitals depends only on the value of n. Hence, different orbitals of the same shell have same energy. However, in case of multielectron atoms, the energies of the orbitals depend upon `n + l` values. Hence, for the same value of n but different value of l, i.e., different subshells belonging to the same main shell have different energies.