[jsmith613]

why does 4s have a lower energy level than 3d when it is empty, but when it is full, 4s has a higher energy level

[Trave11er]

Can you tell, where did you get this information?

[cliverlong]

My guess

Before either are filled, 4s is a lower energy level , albeit with no electrons if the atom is in base state

As we move thorugh the periodic table, Once 4s is filled with electrons, then 3d level starts to fill with electrons

The presence of 3d electrons either (i) pushes the 4s electron level higher or (ii) pushes the 4s electrons further from the nucleus / shields the 4s electrons from the nucleus

Hence 4s electrons end up at a higher energy level than 3d electrons and are lost first upon ionisation of the transition metal

Clive