Each molecular orbital has its own energy level. Chemists sort the MOs by energy levels. Chemists assume that the electrons will fill the lowest energy level MOs first. For example, if a molecule has electrons to fill 15 orbitals, the 15 MOs with the lowest energy levels will be filled. The 15th MO on the list would be called the "highest occupied molecular orbital" (HOMO) and the 16th MO on the list would be the "lowest unoccupied molecular orbital" (LUMO). The difference in the HOMO's energy level and the LUMO's energy level is called the band gap. The band gap can sometimes serve as a measure of the excitability of the molecule: the smaller the energy, the more easily it will be excited. When the electron is excited, it will jump to an unoccupied MO. For example, this can help guess whether something will give out light (luminescence).