3d and 4s subshell

[Emily Hou 1H]

Can someone explain why the 3d subshell has higher energy than the 4s subshell when unfilled but has lower energy than the 4s subshell when filled?

[Grace_Wu]

This is just my understanding hope its right.
Because we fill 4s before 3d, when 3d is unfilled, it has higher energy due to the instability. But when 3d is filled, it has stronger pull towards the nucleus , which makes it more stable. It pushed away 4s from the nucleus, which make 4s unstable and have high energy.

[Shawn Suen]

the d shell has a more stable energy level when filled halfway and fully filled than the s orbital shell. It is an exception when doing the orbitals for copper and chromium.

[Ashley Presnell 1C]

We have two exceptions that cause us to fill up the d orbital halfway/ full before the 4s orbital because a half filled/ full d orbital has a lower energy than a filled 4s orbital, therefore we want to have a more stable electron configuration which is why we have the exceptions of copper and chromium.