Friedrich August Kekule von Stradonitz (how could I not start with him?), in 1865, solved the structure of benzene, the molecule that would become the poster child for the idea of resonating valence bonds. But he had no such thoughts; he deduced the symmetrical six-membered ring from purely chemical data. It wasn’t until 1916 that G. N. Lewis came up with the idea that the valence bond was caused by a shared pair of electrons, one from each atom of the bond; the valence bond was first explained quantum mechanically by Walter Heitler and Fritz London in 1928. They showed that it could be due to the binding of a pair of electrons, one from each atom, by Werner Heisenberg’s exchange interaction. If we have two overlapping atomic orbitals ϕ1 and ϕ2, the exchange integral J defined by E ex = J(S 1 · S...
