Hydrocarbon. Computer graphic of long-chain hydro- carbon molecules floating in space. 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.