Aromaticity

Benzene is conventionally drawn as a hexagon with three alternating double bonds, and that drawing is a known simplification rather than the truth. Every carbon–carbon bond in benzene measures the same length, intermediate between a typical single and double bond — not three short bonds and three long ones. The electrons that the drawing shows as three separate double bonds are actually delocalised: spread evenly around the whole ring in a continuous system above and below the plane of carbons, belonging to the ring as a whole rather than to any one bond.

That delocalisation is what "aromatic" means chemically — nothing to do with smell, despite the name's origin in early observations that many such compounds were fragrant. Spreading the electrons out lowers the molecule's energy well below what three genuine, localised double bonds would have, which is why benzene is markedly more stable than an equivalent non-aromatic triene and resists reactions that would break up the ring system.

That resistance reshapes its chemistry. An ordinary alkene reacts by addition: a double bond opens up and something adds across it, because that trades one pi bond for two new sigma bonds and is downhill in energy. Benzene could react the same way, but doing so would destroy the delocalised system and lose the extra stability aromaticity provides — a cost addition does not pay for a normal alkene. Instead benzene reacts by substitution: one hydrogen is replaced by a new group while the ring itself, and its delocalisation, survives intact. The preference for substitution over addition is the clearest chemical signature that a ring is genuinely aromatic rather than merely drawn to look like one.