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?

NEETprep Answer: During filling up of electrons follow (n+ l) rule. Here 4s has lower energy than 3d orbital.After the orbitals are filled 4s goes beyond 3d,i.e., 4s is farther from nucleus than 3d. So,electron from 4s is removed earlier than from 3d.