Functional group isomerism is a type of isomerism where compounds have the same molecular formula but different functional groups. This means that the atoms are arranged differently, leading to distinct chemical properties and reactions. For example, C2H6O can represent both ethanol, an alcohol, and dimethyl ether, an ether, showcasing how the functional groups influence the compound's behavior. These isomers can exhibit different physical properties, such as boiling points and solubility. The variation in functional groups affects how the molecules interact with other substances, making functional group isomerism an important concept in organic chemistry and the study of organic compounds.