1858 ~ 1861, Kekulé, Couper, and Butlerov, laid the basis for one of the most fundamental theories in chemistry: the structural theory. Valence(化合价): C — tetravalent; O — divalent; H — monovalent
A ring that contains at least one heteroatom (an atom other than carbon) is called a heterocycle, and a substance based on a heterocyclic ring is a heterocyclic compound. Heterocyclic compounds containing N, O or S are by far the most common