Question

What is the first member of the alkene series?

Answer 1

Alkene family is a category of unsaturated hydrocarbons that contain at least one double bond between their carbon atoms. Since a double bond requires the involvement of two carbon atoms, the first member of this series contains two carbon atoms rather than one.

Complete answer:
Carbon is a tetravalent atom that has the special ability to form long chains with itself and also form stable double and triple bonds with other carbon atoms in the chain to satisfy its valency. Based on the type of bonds formed, the hydrocarbons have been categorized into three groups i.e. alkanes, alkenes and alkynes.
Alkanes are saturated hydrocarbons that have only single bond linkages in between their carbon atoms. Alkenes and alkynes are unsaturated hydrocarbons containing double and triple bonds respectively.
Alkenes that have only one double bond throughout the chain have a general formula ${C_n}{H_{2n}}$ where the number of hydrogens attached are always twice the number of carbon atoms present in the hydrocarbon chain.
The first member of the alkene series has only two carbon atoms as it is the minimum number of atoms needed to form a double bond and consequently have four hydrogen atoms. The IUPAC name of this alkene is ethene and its formula is ${C_2}{H_4}$ .
$\Rightarrow$ Thus, the first member of the alkene series is ethene.

Note:
The general formula of ${C_n}{H_{2n}}$ is applicable only in those cases where there is just a single double bond. The position of the double bond in the ring does not alter the formula but changes the IUPAC name of the compound. If there is more than one double bond then the alkenes are called polyenes.