Topic overview

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In the linear combination of atomic orbitals (LCAO) approach, the number of molecular orbitals formed matches the number of atomic orbitals supplied, and each MO holds up to two electrons with opposite spins. Constructive overlap produces lower-energy bonding orbitals; destructive overlap produces higher-energy antibonding orbitals, while non-bonding orbitals retain atomic energy. Bond order is half the difference between bonding and antibonding electron counts, connecting orbital occupancy with bond stability and strength. Electron filling follows the Pauli exclusion principle and Hund’s rule. The lesson also introduces σ and π orbitals, symmetry labels, and s–p mixing in lighter diatomics. Examples such as H₂, He₂, and O₂ illustrate stability and magnetism; delocalized π orbitals help explain benzene’s equivalent C–C bonds.