Hydrocarbon. Computer graphic of a long-chain hydrocarbon molecule stretching across the sea to the horizon. The distorted reflection of the molecule appears on the surface of the water. Hydrocarbons are compounds whose molecules contain only the elements hydrogen and carbon. Long-chain hydrocarbon molecules consist of a single unit (monomer) repeated many times. Plastics, such as polythene (which is made from the monomer CH2CH2), are sometimes made from long- chain hydrocarbon molecules. In polythene, the long chains are tangled together in a solid but flexible mass, giving polythene its strong, pliable quality.