Aromatic hydrocarbons are aromatic organic molecules that form flat ring-shaped bonds. The most simplest of aromatic hydrocarbons are benzene and indole. They can have one ring, "heterocyclic", like benzene or 2 rings, "bicyclic" like napthalene (primary ingredient in mothballs) or many rings, "polycyclic" like anthracene (red dye colourant).