A difficulty arises when we come to potassium. Potassium has an atomic number of 19. For potassium and calcium, the 4s orbital dips in energy below the 3d orbitals, and so electrons go into the 4s ...

We find that it is the 4s orbital electrons that are ionised first before the 3d orbitals. We have to conclude that, ironically for the d-block elements, the 4s orbitals are in fact higher in energy ...