"Bohr proposed in the first place that the energies of atoms are quantized, in the sense that the atom exists in only a discrete set of states, with energies (in increasing order) E1, E2, . . . . The frequency of a photon emitted in a transition m → n or absorbed in a transition n → m is given by Einstein’s formula E = hν and energy conservation by \nu=(E_m-E_n)/h. … Bohr’s formulas could be used not only for single-electron atoms, like hydrogen or singly ionized helium, but also roughly for the innermost orbits in heavier atoms, where the charge of the nucleus is not screened by electrons, and we can take Ze as the actual charge of the nucleus."