Define functional group and explain its general characteristics and uses.

(N/A) Definition: $A$ functional group is an atom or a group of atoms bonded together in a unique manner,which is usually the site of chemical reactivity in an organic molecule.
Characteristics: Compounds containing the same functional group undergo similar chemical reactions. For example,$CH_{3}OH$,$CH_{3}CH_{2}OH$,and $(CH_{3})_{2}CHOH$,all containing the $-OH$ functional group,liberate hydrogen gas upon reaction with sodium metal.
Uses: Functional groups allow for the systematic classification of organic compounds into different classes based on their chemical behavior and structural features.