HOMO and LUMO
The highest occupied and lowest unoccupied molecular orbitals, which together govern a molecule's reactivity and spectroscopy.
Formula / equation
ΔE = E_LUMO − E_HOMO
Unit
eV
In depth
These frontier orbitals dominate chemical interactions: a nucleophile donates from its HOMO into an electrophile's LUMO. The energy gap between them sets the wavelength of the lowest electronic absorption, so a small gap means visible absorption and hence colour, and also correlates with semiconductor band gaps.
Examples in the real world
Conjugated dyes absorb visible light because extended conjugation shrinks the HOMO–LUMO gap below 3 eV.