Ethene is a member of the alkenes homologous series. Describe the structure of ethene and explain why alkenes, unlike alkanes, can undergo addition reactions to form polymers.

Ethene has the formula C₂H₄ and consists of two carbon atoms joined by a double bond, each carbon also bonded to two hydrogen atoms.

Because the C=C bond is a double bond, ethene is unsaturated. The π‑bond can be broken, allowing other atoms or molecules to add across the former double bond.

When many alkene monomers undergo such addition reactions, they link together to give a polymer chain.

Alkanes, in contrast, are saturated and contain only single C–C bonds; they have no π‑bond to break, so they cannot undergo addition reactions and therefore cannot polymerise in this way.