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Why are alkenes called unsaturated hydrocarbons?

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structural formulas of some organic compounds — organic compound The structures of organic compounds can be depicted in condensed, expanded, and three-dimensional structural formulas.

Alkenes are classified as unsaturated hydrocarbons because they contain at least one carbon-carbon double bond, which consists of a strong sigma ( $σ$ ) bond and a more reactive pi ( $π$ ) bond.123

The term "unsaturated" refers to the ability of these molecules to undergo addition reactions. Because the pi bond is a region of high electron density, it can be broken to allow the addition of other atoms—such as hydrogen—to the carbon atoms, resulting in the formation of new, stronger sigma bonds.24

In contrast, alkanes are described as "saturated" because they contain only single bonds and cannot be transformed by addition reactions into molecules with a greater number of sigma bonds. Essentially, alkenes are "unsaturated" because they have not reached the maximum possible number of atoms that can be bonded to their carbon framework.245