Question

energy level diagram for Ni2+
why the electrons are taken from s orbital but not d orbital?

Answer 1

This is one of the exceptions to Aufbau's Principle, which unfortunately has no sound explanation. One idea is that the 4s electrons have less energy than the 3d electrons, but that is paradoxical because the 4s orbital comes before the 3d orbital.

Answer 2

Are there any other exceptions?

Answer 3

Yes, for transition metals, electrons often move from lower orbitals to higher orbitals to have have filled sub-shells. This is also an explanation for a few of the oxidation states. Having half filled or completely filled sub-shells make the metals more stable.

Answer 4

ok thank you so much!