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?

Question

Philoid · Accepted Answer

Electrons are filled according to the n+l rule. If an orbital has lower n+l value, then electron will enter that orbital.

For 3d, n+l= 3+2=5

4s, n+l= 4+0=4

So, electron will first enter 4s and then 3d while filling.But, 4s electrons are loosely held by the nucleus and are outside 3d, so removing a 4s electron(ionization of the atom) becomes easier than removing a 3d electron.