For two molecules to be functional isomers, they must contain key groups of atoms arranged in particular ways. Some of the best examples come from organic chemistry. CHO is a molecular formula. Depending on how the atoms are arranged, it can represent two different compounds dimethyl ether CH-O-CH or ethanol CHCH-O-H. Dimethyl ether and ethanol are functional isomers. The first is an ether. The carbon chain-oxygen-carbon chain functionality is called an ether. The second is an alcohol. The carbon chain-oxygen-hydrogen functionality is called an alcohol.