Steve Marsden’s

The Schrödinger orbitals we have discussed are actually for the single electron in the hydrogen atom. As the electron is given more energy successive orbitals are "created" while the lower energy ones temporarily cease to exist.

It is possible to arrange atomic orbitals in a kind of order by increasing energy, following the sequence of hydrogen electron excitation. Such an order shows that sub-levels for the hydrogen atom are degenerate, that is, they have identical energies.

But this order is only true for a single-electron system. In a many-electron system ("many" is more than one!) the situation is more complex. Repulsions and interactions among electrons cause a slight separation of the sub-levels and a significant overlap of energy levels beginning with n=3.

In a many-electron atom the sub-level energies always increase in the same way: s orbitals being the lowest energy in each level, followed by p orbitals, then d and finally f.