Alkenes are another homologous series of Hydrocarbons - they contain only Carbon and Hydrogen atoms.

The difference between Alkenes and Alkanes is that Alkenes contain a double bond - their functional group - the part of the molecule where reactions will happen.

We say they are unsaturated - more atoms could bond to the Carbon chain - although if an examiner asks - unsaturated means "contains a C=C" bond.

Because a C=C bond is needed to be an Alkene, it isn't possible to have an Alkene shorter than two Carbons in length (Ethene).

We only have to draw the displayed formulae of the first four Alkenes.

Notice that the start of their names are the same as in Alkanes, differing only by ending with -ene instead of -ane.

And that, unlike Alkenes, there is no Methene because you can't have an Alkene with only one Carbon atom since they must contain the C=C bond

Also notice that each Alkene differs from its neighbours by -CH₂-

Because there are fewer Hydrogens than in Alkanes the general formula must be different.

C₂H₄, C₃H₆, C4H8, C₅H₁₀ etc ----- The number of H's is always double the number of C's.

So, the general formula is C_nH_2n.