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

For 3d - orbital, `n+l = 3+2=5` and for 4s - orbital `n+l =4+0=4`. So 4s - orbital has lower energy than 3d - orbital and hence, 4s - orbital is filled before 3d - orbital. But 4s - orbital is at a larger distance from the nucleus than 3d - orbital and so in the ionisation process electrons are removed from 4s - orbital prior to 3d - orbital.