In science, a compound is a pure chemical substance made up of two or more different chemical elements that are chemically bonded together in a fixed ratio. When the elements come together, they react with each other and form strong bonds that require significant energy to break apart. The elements dont retain their individual properties, nor does the compound necessarily behave like those elements.

Compounds can be broken down into their constituent elements by chemical changes, which are changes in which the organization of the atoms is altered. Chemical compounds show a bewildering array of characteristics. At ordinary temperatures and pressures, some are solids, some are liquids, and some are gases. The colors of the various compounds span those of the rainbow. Some compounds are highly toxic to humans, whereas others are essential for life.

Compounds are sometimes distinguished by whether theyre made up of molecules. A molecule is a structure that consists of two or more atoms that are chemically bonded together.