Ionization of Transition Metals

[Chocolatebear89]

I understand that 3d orbital has higher energy than 4s orbital, and we remove and electron from a transition metal, you usually remove from 4s orbital first. I was wondering why?

Doesn't 3d orbital have higher energy, because it is further away from nucleus, which makes it more likely to remove?

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[Rabolisk]

My understanding is that a free 4s orbital is lower energy than a free 3d orbital. However, when there are already electrons in both orbitals, the 4s orbital becomes higher in energy, and thus the first to go when ionized. I don't quite know the reason for this.

 

[vsl5]

Actually, I dont think there is a hard rule for transition metals which makes it a lot trickier. Some of them have d orbitals that are degenerate with the 4s orbital (same energy level). In some cases, it is better to have all half filled orbitals like 3d5 via Hund's rule. Supposedly this makes it more stable than say 4s2 3d3 and this explains why some orbitals can lose 4s electons before 3d ones even though 4s is lower in energy. Others can be different where the 3d orbital is at a higher energy level than the 4s orbitals.