Molecular orbital Qualitative approach HOMO and LUMO
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 difference in the HOMO's energy level and the LUMO's energy level is called the band gap.