Where the localized π breaks. Put sp² carbons in a ring and their leftover p orbitals don't pair off into separate double bonds — they all overlap at once into a single delocalized cloud above and below the ring. Benzene has no double bonds in the cartoon sense: all six C–C bonds are identical (139 pm), the electrons belong to the whole ring, and the two Kekulé structures with alternating doubles are just the localized picture confessing that the truth is the average. Spreading six electrons over a six-atom box lowers the energy — the same trick as the metal's sea — buying ~150 kJ/mol of extra stability when the count is right: Hückel's 4n+2.
| C–C bonds | all 139 pm (single 154, double 134) |
| shape | planar regular hexagon, 120° |
| stabilization | ≈ 150 kJ/mol below "3 double bonds" |
| behaviour | substitutes, won't add (keeps the ring) |