The honest picture under every cartoon. Lewis structures, hybrids, and resonance arrows all speak a localized language — bonds that live between two atoms — and they patch over delocalization by drawing several structures and averaging. MO theory drops the patch: atomic orbitals combine into molecular orbitals that span the whole molecule (N orbitals in, N orbitals out), filled lowest-first. Each combination is either bonding (in-phase, density between nuclei) or antibonding (out-of-phase, a node between), and the count of filled bonding minus antibonding is the bond order. This is what predicts the things the cartoons get wrong — that O₂ is magnetic, why benzene's magic number is 4n+2, why methane shows two ionization energies, not four.
Resonance is the localized model confessing: when one Lewis structure won't do, draw several and call the molecule their average. The arrow ↔ is not the molecule flipping — it means neither drawing is real.
MO theory never localizes in the first place. The resonance hybrid ≈ the filled bonding MOs; both describe one electron density, but MO handles delocalization, magnetism, and spectra directly.
| O₂ magnetic | 2 e⁻ in degenerate π* → unpaired → paramagnetic (Lewis says paired) |
| He₂, bond 0 | σ + σ* both full → cancels |
| 4n+2 | closed bonding shell at 2,6,10 (Frost circle) |
| colour | HOMO→LUMO gap = light absorbed |