molecular orbital theory
/ MO theory /
The simple picture of a bond as two atoms sharing a pair of electrons in the space between them works wonderfully, but it is a cartoon. Molecular orbital theory is the deeper picture: when two atoms come together, their atomic orbitals merge into new orbitals that belong to the whole molecule, not to either atom alone. Electrons no longer sit on individual atoms; they occupy these shared molecular orbitals that span the bond.
The central idea is that orbitals combine in two ways. When two atomic orbitals add together in phase, they reinforce in the region between the nuclei, building up electron density right where it glues the atoms — this is a bonding molecular orbital, lower in energy and stabilizing. When the same two orbitals combine out of phase, they cancel between the nuclei, leaving a node (a gap of zero electron density) there — this is an antibonding orbital, higher in energy and destabilizing, usually marked with a star (sigma-star, pi-star). Electrons fill the low-energy bonding orbitals first. A bond exists when more electrons end up in bonding than in antibonding orbitals; the net 'bond order' is half that difference.
MO theory matters because it explains things the simple shared-pair model cannot. It accounts for why oxygen gas is magnetic (it has unpaired electrons), why some molecules conduct or absorb particular colours of light (electrons jumping from a filled bonding orbital to an empty antibonding one), and it underlies the modern understanding of conjugation, aromaticity and UV-visible spectra. The trade-off is honesty about complexity: full MO theory is mathematical, so organic chemists usually use the simpler hybridization and resonance pictures for everyday work and reach for MO theory when those pictures fall short.
In H2, the two hydrogen 1s orbitals combine into a low-energy bonding sigma orbital (which holds both electrons) and an empty high-energy antibonding sigma-star orbital. Both electrons in bonding, none in antibonding, gives a bond order of 1.
Two atomic orbitals in, one bonding and one antibonding orbital out; filling the bonding one makes the bond.
Antibonding orbitals are real and always created alongside bonding ones; a bond exists only because the bonding orbitals are more occupied. MO theory complements, not replaces, the everyday hybridization picture.