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

During filling up of electrons follow Aufbau's `(n+l)` rule. Here `4s` `(4+0=4)` has lower energy than `3d` `(3+2=5)` 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`.