Ethanol and dimethyl ether have the same molecular formula, C2H6O. But they have different properties. Ethanol is a liquid that mixes with water and boils at 78 degrees Celsius. Dimethyl ether is a gas that is only slightly soluble in water and boils at minus 24 degrees Celsius. The difference is the arrangement of atoms. Ethanol has a hydroxyl group (-OH). Dimethyl ether has an ether group (-O-). The functional group determines the properties.

A functional group is a specific group of atoms responsible for a molecule's characteristic reactions. It is the reactive part of the molecule. The rest of the molecule, called the carbon skeleton, affects solubility, boiling point, and size, but the functional group determines the chemistry. Common functional groups include hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, and methyl. Each has its own reactivity.

Functional groups are the basis of organic chemistry. They classify molecules into families: alcohols, aldehydes, ketones, carboxylic acids, amines, thiols, and others. They determine how molecules interact with each other and with biological systems. They are the sites of chemical reactions in metabolism, drug action, and materials synthesis.

Identifying functional groups is a key skill in organic chemistry. Spectroscopy, especially infrared and nuclear magnetic resonance, reveals which groups are present. The infrared spectrum shows characteristic absorption bands for each group. The NMR spectrum shows the environment of each hydrogen or carbon atom. Together, they reveal the structure of a molecule.