The alkenes and their formulae

1. Ethene is $C_2{H}_4$, the smallest alkene.
2. Propene is $C_3{H}_6$.
3. Butene is $C_4{H}_8$.
4. Each fits the general formula $C_n{H}_{2n}$.
5. An alkene has two fewer hydrogens than the alkane with the same number of carbons.

The double bond is the functional group

1. Every alkene contains a carbon to carbon double bond, written $C=C$.
2. That double bond is the alkene functional group.
3. Such a group is what gives a family its characteristic reactions.
4. The rest of the molecule is a chain of carbons carrying hydrogen atoms.
5. Every alkene reacts in a similar way because every one contains the same group.

Drawing the structures

1. Each carbon still forms four bonds in total, with the double bond counting as two of them.
2. Ethene is two carbons joined by a double bond, each carrying two hydrogens.
3. Propene is three carbons, with the double bond between the first and second.
4. The carbon at the end of the double bond carries two hydrogens, and the middle one carries a single hydrogen.
5. The third carbon in propene is joined by a single bond and carries three hydrogens.

Alkenes are unsaturated

1. A molecule containing a carbon to carbon double bond is unsaturated.
2. Unsaturated means the molecule could hold more hydrogen than it does.
3. The double bond can open so that two more atoms join on.
4. An alkane cannot do this, since it has no double bond to open.
5. This difference is what makes alkenes far more reactive than alkanes.