the structural features that make it possible to classify compounds into families are called functional groups. A functional group is a group of atoms that has a characteristic chemical behavior in every molecule where it occurs. For example, compare ethylene, a plant hormone that causes fruit to ripen,with menthene, a much more complicated molecule. Both substances contain a carbon-carbon double-bond functional group, and both therefore react with Br2 in the same way to give products in which a Be atom has added to each of the double-bond carbon (Figure 3.1). This example is typical