Why are alkenes classified as unsaturated hydrocarbons?

Alkenes are classified as unsaturated hydrocarbons because they have fewer hydrogen atoms compared to alkanes, which are their saturated counterparts. This reduction in hydrogen atoms occurs due to the presence of at least one carbon-carbon double bond in alkenes.

For example, the simplest alkene, ethene (C2H4), has a double bond between the carbon atoms, which means it can bond with fewer hydrogen atoms than ethane (C2H6), which has only single bonds and is fully saturated. This characteristic allows alkenes to participate in additional chemical reactions, such as polymerization and addition reactions, unlike saturated hydrocarbons which cannot.

The other options do not accurately describe why alkenes are classified as unsaturated. The assertion that they have no hydrogen atoms is incorrect because alkenes do indeed contain hydrogen atoms, just in lesser amounts than alkanes. The claim that they contain only single bonds is also incorrect, as the defining feature of alkenes is the presence of at least one double bond, not single bonds. Finally, alkenes are not necessarily solid at room temperature; in fact, many alkenes are gases or liquids depending on their molecular size. Hence, the correct identification of