A compound is a substance made up of two or more different chemical elements combined 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 are sometimes confused with mixtures, but the two differ in important ways. In a mixture, substances are combined, but theres no chemical reaction or bonding like that found in compounds. The individual substances in a mixture retain their own properties and can be separated into their original components.

A compound is identified by its name and chemical formula. The formula reflects the compounds elemental makeup. It includes the chemical symbol for each element in the compound, and the number of atoms for that element if theres more than one. Compounds can be molecular compounds held together by covalent bonds, salts held together by ionic bonds, intermetallic compounds held together by metallic bonds, or the subset of chemical complexes that are held together by coordinate covalent bonds.

Examples of compounds include water, table salt, and baking soda. 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.