General formula for alkenes.

## The reasoning Alkenes are hydrocarbons with **one carbon-carbon double bond** (C=C). This double bond is the key to their formula. Start with alkanes (single bonds only): their formula is CₙH₂ₙ₊₂. For example, ethane is C₂H₆. Now, when you introduce a double bond, you're essentially **removing two hydrogen atoms** to create that C=C bond. Think of it: instead of C-C (each carbon bonded to more hydrogens), you have C=C (sharing two bonds between them, so fewer hydrogens needed). So: CₙH₂ₙ₊₂ − 2H = **CₙH₂ₙ** Test it: Ethene (simplest alkene) is C₂H₄ → Using n=2: C₂H₂₍₂₎ = C₂H₄ ✓ ## Why the wrong options tempt you **A) CₙH₂ₙ₊₂** — This is for alkanes (no double bonds). Easy to confuse if you forget the double bond reduces hydrogen count. **C) CₙH₂ₙ₋₂** — This is for alkynes (triple bonds, C≡C). Students mix up "enes" and "ynes." **D) CₙH₂ₙ₋₁** — Not a standard hydrocarbon formula; just a distractor. ## Quick takeaway **"Alkenes = one double bond = lose 2H from alkane formula = CₙH₂ₙ."** Remember: alk**ENE** = **2N**.