lowest unfilled molecular orbital

The lowest unfilled molecular orbital refers to the lowest-energy molecular orbital (MO) that does not contain any electrons. In molecular orbital theory, electrons occupy MOs, which are wave functions describing electron distribution across all nuclei in a molecule. These MOs are formed from combinations of atomic orbitals.1

When constructing MOs, bonding orbitals have enhanced electron density between nuclei, lowering energy, while antibonding orbitals have reduced density, increasing energy. Electrons fill these orbitals starting from the lowest energy level, following the Pauli exclusion principle, which limits each orbital to two electrons. The lowest unfilled MO is the next available energy level for electrons. This concept is crucial for understanding chemical bonding and molecular properties.123