A
molecule (ˈmɒlɪkjuːl) is an electrically neutral group of two or more
atoms held together by
covalent chemical bonds. Molecules are distinguished from
ions by their lack of electrical charge. However, in
quantum physics,
organic chemistry, and
biochemistry, the term
molecule is often used less strictly, also being applied to
polyatomic ions.
In the
kinetic theory of
gases, the term
molecule is often used for any gaseous particle regardless of its composition. According to this definition,
noble gas atoms are considered molecules despite being composed of a single non-bonded atom.
A molecule may be
homonuclear, that is, it consists of atoms of a single
chemical element, as with
oxygen (O
2); or it may be a
chemical compound composed of more than one
element, as with
water (H
2O). Atoms and complexes connected by non-covalent bonds such as
hydrogen bonds or
ionic bonds are generally not considered single molecules.
Molecules as components of matter are common in organic substances (and therefore biochemistry). They also make up most of the oceans and atmosphere. However, the majority of familiar solid substances on Earth, including most of the minerals that make up the
crust,
mantle, and
core of the Earth, contain many chemical bonds, but are
not made of identifiable molecules. Also, no typical molecule can be defined for
ionic crystals (
salts) and covalent crystals (
network solids), although these are often composed of repeating
unit cells that extend either in a
plane (such as in
graphene) or three-dimensionally (such as in
diamond,
quartz, or
sodium chloride). The theme of repeated unit-cellular-structure also holds for most condensed phases with
metallic bonding, which means that solid metals are also not made of molecules. In
glasses (solids that exist in a vitreous disordered state), atoms may also be held together by chemical bonds without presence of any definable molecule, but also without any of the regularity of repeating units that characterises crystals.