Atomic Energy Levels
The relative energy values of all electrons in the atom involved an electron transition from different subshells resulting in radiations of different wavelengths. The first principle shell has a single subshell, designated as the 1s shell, the second principal shell has two subshells, designated as 2s and 2p. The subshells are further divisible into orbitals, with each orbital containing a maximum of two electrons. The energies corresponding to the various subshells of any given shell increase in the order s<p<d<f.
In some cases the d or f subshells in a shell of lower principal quantum number overlap with s or p subshell in the next higher shell. In certain atoms their order of occupancy is reversed, giving rise to a number of minor exceptions to the expected order of filling orbitals.