4s orbital lower in energy than 3d

[Jasmine Emtage-1J]

I know we went over this in class on Friday, but I'm still unsure why the 3d state is higher in energy than the 4s state for Potassium and Calcium. Can someone explain why?

[Nahelly Alfaro-2C]

The 3d state is higher in energy than the 4s state for potassium and calcium because Z(atomic number) is equal to 20. In addition, the d-electron are higher in energy than the p-electron on the same shell which results in higher energy than s-electron of the same shell too.

[Cameron Smith 1I]

If the 3d state is higher in energy then why is it that we write it first before the 4b orbital in the electron configuration?

[Chem_Mod]

For K and Ca atoms, the 3d orbitals are higher in energy, but for Z > 20, the order switches as 3d is lower than 4s. This is a results of complex nuclear electrons interactions as well as electron-electron interactions due to resulting changes in the number of protons and the electrons. That is why you see 3d first than 4s for the transition metals. K and Ca do not have 3d electrons in the ground state, so you don't have issue writing 3d for them.