The general formula for alkenes is:

## 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 (which is C₂H₂₍₂₎₊₂). Now, when you introduce a double bond, you **remove two hydrogen atoms** because two carbons now share four electrons instead of two. So from CₙH₂ₙ₊₂, subtract 2H → **CₙH₂ₙ**. Check it: Ethene (simplest alkene) = C₂H₄ (fits C₂H₂₍₂₎ ✓). Propene = C₃H₆ (fits C₃H₂₍₃₎ ✓). ## Why the wrong options tempt you **A) CₙH₂ₙ₊₂** — This is alkanes (no double bonds), like petrol or cooking gas. **C) CₙH₂ₙ₋₂** — This is alkynes (triple bond C≡C), like the gas in welding torches. One more degree of unsaturation means two more H atoms lost. **D) CₙH₂ₙ₋₁** — This doesn't match any major hydrocarbon family. It's a distractor. ## Quick takeaway **Each double bond costs you 2 hydrogens**: alkanes are CₙH₂ₙ₊₂, alkenes lose 2H → CₙH₂ₙ, alkynes lose another 2H → CₙH₂ₙ₋₂.