Question

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?

Answer 1

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

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

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

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