Carbon atoms normally try to form four single bonds with other atoms. In unsaturated hydrocarbons, some carbon atoms form multiple bonds with each other instead. This happens because the molecule does not have enough hydrogen atoms to fill every available spot.
The double and triple bonds contain special chemical connections called pi bonds. These pi bonds are much weaker than normal single bonds. This weakness makes unsaturated hydrocarbons very chemically reactive.
They easily take part in processes known as addition reactions. In an addition reaction, the double or triple bond breaks open. New atoms then attach directly to the carbon atoms without kicking any other atoms out.
Chemists typically group these reactive molecules into three main structural families. Alkenes contain at least one double bond and have the general formula CnH2n. Alkynes contain at least one triple bond and use the formula CnH2n-2.
Aromatic hydrocarbons contain a special stable ring of alternating double bonds. These reactive molecules are vital building blocks in chemical factories. They are heavily used to make plastics, everyday medicines, and synthetic rubber.