under which circumstances does 4s have lower energy than 3d?

[vuongnaomi1L]

So to my understanding, the 4s state is lower than the 3d state only when the 3d state does not get filled, or when the 4s state gets filled before the 3d state, correct?

For example, in the electron configuration of Ca:[Ar] 4s^2, the 4s state would be lower than the 3d state because Ca does not have any electrons in the 3d state.

[Katie Lam 2J]

I believe that is correct. In one of the lectures, Dr. Lavelle mentioned that, for multi-electron atoms after atomic number 20 (Ca), the 4s state is higher in energy than 3d. Hope this helps!

[Ivy Tan 1E]

Hi!
Yes, you're right. The only circumstance in which the 4s state has less energy than the 3d state is when the d orbitals are empty. This only applies to elements K and Ca in period 4. For all elements in period 4 from groups 3-18, the 4s state has more energy than the 3d state because these elements have electrons in the 3d orbitals. Hope this helped!!