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Why are alkenes and alkynes called unsaturated hydrocarbons?

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structures of common hydrocarbon compounds — hydrocarbon Structures assumed by hydrogen (H) and carbon (C) molecules in four common hydrocarbon compounds.

Alkenes and alkynes are classified as unsaturated hydrocarbons because they contain multiple carbon-carbon bonds, which distinguishes them from saturated hydrocarbons like alkanes that contain only single bonds.12

In chemistry, the term "saturated" refers to a molecule in which the carbon atoms are bonded to the maximum possible number of hydrogen atoms. Because alkenes contain at least one carbon-carbon double bond and alkynes contain at least one carbon-carbon triple bond, they possess fewer hydrogen atoms than a corresponding alkane with the same number of carbon atoms. These multiple bonds are formed by sharing additional electron pairs between carbon atoms, which reduces the capacity of the molecule to bond with additional hydrogen atoms, thus rendering them "unsaturated".231