While filling up of electrons in the atomic orbitals, the 4s-orbital is filled before the 3d-orbital but reverse happens during the ionisation of the atom. Explain why ?

Explanation

According to n + 1 rule : For 3d = n + 1 = 5 , 4s = n + 1 = 4
Therefore, the electron will enter in 4s orbital first and then in 3d orbitals. Ionisation enthalpy is responsible for the ionisation of atom. 4s electrons are loosely held by the nucleus. So electrons are removed from 4s orbital prior to 3d