So far we have said very little about the energies of these quantum states of an atom. The energy depends mainly on the principal quantum number, n, and to a lesser extent on the orbital-shape quantum number, l. Within a given n state, a higher I value corresponds to a higher energy for atoms with more than one electron. This I dependence can be thought of as producing a splitting of the principal quantum levels. If this splitting were zero, then wave mechanics would reduce to the old Bohr theory, with the same expression relating energy, E, to principal quantum number, n. (This is true only for hydrogen.) If the splitting were small, so the upper states from one n level did not overlap with the lower states of the next, then the energy levels would appear as opposite on the blue background. The sequence of levels of increasing energy would be very orderly: 1s ; 2s, 2p ; 3s, 3p, 3d ; 4s, 4p, 4d, 4f ; 5s, - - .