Functional isomers are one of the structural isomers. Two compounds in functional isomerism have the same molecular formula (the number of each atom is the same, like cyclohexane: and 1-hexene:). But, the atoms are connected in other ways so that the groupings are different. We call these groups of atoms functional groups, functionalities, or moieties. Another way to say this is that two compounds that have the same molecular formula, but have different functional groups, are functional isomers.