Clarification of 4s/3d energy

[Ysabelle Magat 1E]

So Lavelle said that "for multi-electron atoms after atomic number (Z) 20, 4s state is higher in energy than 3d".

Does this mean for potassium and calcium, 3d is higher than 4s? And what exactly is the reason for this?

[Reza Hemmati 3L]

Hi!

Potassium and calcium don't have any electrons in the 3d sublevel, their highest-energy electrons/valence electrons are in the 4s sublevel. From what I understand, before Z = 20(up to calcium), 4s has lower energy than 3d, which causes the valence to settle in 4s at ground state(not excited). However, once electrons start filling up the 3d sublevel(Z = 21 and beyond), those electrons repel the 4s sublevel to be farther out from the nucleus, which effectively increases the energy of 4s. This results in 4s being lower in energy than 3d before Z > 20 and 4s being higher in energy than 3d after Z > 20.

Hope this helps! ^^