Aromatic compounds are carbon-framework molecules in which the carbon skeleton is held together in flat rings by o- bonds, and the carbon p orbitals perpendicular to the rings are involved in extensive delocalized electron systems. Benzene is the simplest such molecule, but many rings can be fused together in larger molecules. Naphthalene, CH, has two fused rings; anthracene, CH has three, and many larger molecules exist (see right). Replacing the peripheral hydrogens by other chemical groups gives rise to a rich and varied branch of organic chemistry, which includes many biologically important molecules, flavorings, dyestuffs, light receptors, and carcinogenic (cancer-producing) agents.