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 orbital before occupying the 3d orbitals.

The electron configurations of the 3rd period d-block elements are shown below: The electron configurations for chromium and copper seem anomalous as the 4s orbital is not doubly occupied. However, we ...