Alkenes are unsaturated open-chain hydrocarbons that contain at least one carbon–carbon double bond (C=C). The presence of this double bond is what makes them 'unsaturated' — the carbon atoms are not bonded to the maximum possible number of hydrogen atoms, so alkenes can undergo addition reactions (for example, adding H₂, Br₂, or HX across the double bond).

The general molecular formula of alkenes is CₙH₂ₙ, valid for n ≥ 2 (a double bond needs at least two carbon atoms). This formula can be understood by comparison with the saturated alkane CₙH₂ₙ₊₂: introducing one C=C double bond means forming one extra bond between two carbons, which removes two hydrogen atoms from the alkane count, giving CₙH₂ₙ.

Worked check with n = 2: 2n = 4, so the formula predicts C₂H₄, which is exactly ethene, the simplest alkene. For n = 3, C₃H₆ is propene, and for n = 4, C₄H₈ is butene — all consistent with CₙH₂ₙ.

The other choices correspond to different families or are not valid hydrocarbon series:

• CₙH₂ₙ₊₂ is the general formula of alkanes (fully saturated, single bonds only), not alkenes.
• CₙH₂ₙ₋₂ is the general formula of alkynes (one C≡C triple bond) or of cyclic alkenes, so it describes a more unsaturated class.
• CₙHₙ does not represent any standard homologous series of hydrocarbons and gives impossible structures for small n.