Why the 4s Orbital Is Occupied before the 3d

It is demonstrated that the difference between the total energies of two electronic configurations can be expressed as an orbital energy difference, provided that the configurations differ only in the assignment of one electron and that each of the energies of the differentiating orbitals corresponds to an ionization energy in its own configuration. With orbital energies defined this way, there is nothing paradoxical about 4s being preferred to 3d in the transition series. The vacant 3d orbitals are not available at a lower energy than the occupied 4s.