3d and 4s

[Emmaraf 1K]

Do we know the reason why the 4s subshell is a lower energy than the 3d UNTIL the 3d subshell starts to be filled. What causes the switch in energy to occur?

[Danielle_Gallandt3I]

So I think part of the reason might be that when there are two electrons in the same orbital (one with a spin up, one with a spin down) that is at a higher energy state because the electrons repel each other. By that logic, once the 4s orbital is filled (with the two electrons repelling each other) then that means it would be at a higher energy state, which could be part of the reason as to why the 3d orbital is then considered lower energy only after the 4s orbital is filled.

[Jonas Talandis]

I think it has something to do with electron affinity and inter-molecular forces, which we haven't learned about yet. But for know, I think we just need to understand that 4s comes is filled before 3d except for when the d orbital steals an e- from the 4s for Cr and Cu.