n.
Any substance composed of identical molecules consisting of atoms of two or more elements .
Millions are known, each unique, with unique properties. Most common materials are mixtures of compounds. Pure compounds can be obtained by physical separation methods, such as precipitation and distillation . Compounds can be broken down into their constituents to various degrees or changed into new compounds by chemical reactions . Atoms always combine into molecules in fixed proportions, distinguishing compounds from solutions and other mechanical mixtures. Compounds are often classified as inorganic and organic compounds ; coordination complexes, which contain metal atoms (usually transition elements ) bonded to ligands that may be organic, are somewhat in between. Compounds may also be classified by whether they have ionic or covalent bonds (many include both types).