In science, 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 chemical bonds that require significant energy to break apart. 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.

Chemical compounds have a unique and defined chemical structure held together in a defined spatial arrangement by chemical bonds. They 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.

It is important to note that a molecule consisting of atoms of only one element is not a compound. Compounds can be transformed into a different substance by a chemical reaction, which may involve interactions with other substances. In this process, bonds between atoms may be broken and/or new bonds formed.