A functional group is a specific group of atoms that gives a compound its characteristic chemical and physical properties, and determines its reactivity and behavior in chemical reactions. It is a part of the molecule that undergoes a predictable and characteristic set of reactions, regardless of the rest of the molecule.

Functional groups are groups of atoms that are often found in organic molecules, such as alcohols, aldehydes, ketones, carboxylic acids, esters, amines, amides, halides, and ethers. These groups of atoms are responsible for the chemical and physical properties of the molecule, and they determine how the molecule interacts with other molecules.

For example, the hydroxyl functional group (-OH) is found in alcohols, and gives them their characteristic properties, such as the ability to form hydrogen bonds and to act as weak acids. The carbonyl functional group (>C=O) is found in aldehydes, ketones, and carboxylic acids, and gives them their characteristic reactivity, such as the ability to undergo nucleophilic addition reactions. The amine functional group (-NH2) is found in amines and amides, and gives them their characteristic basicity and ability to form hydrogen bonds.