Benzene C6H6: consists of six carbon atoms arranged in a ring in the form of a regular hexagon. From each carbon atom (black spheres drawn at the covalent radius of carbon) projects a hydrogen atom (green spheres drawn at the covalent radius of hydrogen). The carbon and hydrogen atoms all lie in the same plane. It is an aromatic hydrocarbon (or arene). Delocalized electrons exist above and below the plane of the ring (shown as a red cloud above and below the benzene ring). These are the pi (π) electrons from the π orbitals. Benzene is carcinogenic.