Functional group isomers are compounds that have the same molecular formula but differ in the type of functional group present. This means that while they contain the same number of atoms of each element, the arrangement and types of bonds lead to different chemical properties and reactivities. For example, C2H6O can exist as both ethanol and dimethyl ether, showcasing how the functional group influences the compound's behavior. These isomers are important in organic chemistry because they can exhibit distinct physical and chemical properties despite having identical molecular formulas. Understanding functional group isomerism helps chemists predict how different compounds will react in various chemical reactions, which is crucial in fields like pharmaceuticals and materials science.