In chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds. According to this definition noble gases would also be considered molecules despite the fact that they are composed of a single non-bonded atom.

History
In a molecule, at least two atoms are joined by shared pairs of electrons in a covalent bond. It may consist of atoms of the same chemical element, as with oxygen (O₂), or of different elements, as with water (H₂O). Atoms and complexes connected by non-covalent bonds such as hydrogen bonds or ionic bonds are generally not considered single molecules.
No typical molecule can be defined for ionic (salts) and covalent crystals (network solids) which are composed of repeating unit cells that extend either in a plane (such as in graphite) or three-dimensionally (such as in diamond or sodium chloride).
The science of molecules is called molecular chemistry or molecular physics, depending on the focus. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with the laws governing their structure and properties. In practice, however, this distinction is vague. In molecular sciences, a molecule consists of a stable system (bound state) comprising two or more atoms. Polyatomic ions may sometimes be usefully thought of as electrically charged molecules. The term unstable molecule is used for very reactive species, i.e., short-lived assemblies (resonances) of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, Van der Waals complexes, or systems of colliding atoms as in Bose-Einstein condensates.

Overview
Most molecules are far too small to be seen with the naked eye, but there are exceptions. DNA, a macromolecule, can reach macroscopic sizes, as can molecules of many polymers. The smallest neutral molecule is the diatomic hydrogen (H₂), with an overall length of roughly twice the 74 picometres (0.74 Å) bond length. Molecules commonly used as building blocks for organic synthesis have a dimension of a few Å to several dozen Å. Single molecules cannot usually be observed by light (as noted above), but small molecules and even the outlines of individual atoms may be traced in some circumstances by use of an atomic force microscope. Some of the largest molecules are supermolecules.

Molecular size
The empirical formula of a molecule is the simplest integer ratio of the chemical elements that constitute the compound. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely - dimethyl ether has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers. The empirical formula is often the same as the molecular formula but not always. For example the molecule acetylene has molecular formula C₂H₂, but the simplest integer ratio of elements is CH. The molecular formula reflects the exact number of atoms that compose a molecule.
The molecular mass can be calculated from the chemical formula and is expressed in conventional atomic mass units equal to 1/12th of the mass of a neutral carbon-12 (C isotope) atom. For network solids, the term formula unit is used in stoichiometric calculations.

Molecular geometry

Main article: Spectroscopy Theoretical aspects

Molecular modelling
Covalent bond
Diatomic molecule
History of the molecule
Molecular geometry
Molecular Hamiltonian
Molecular orbital
Nonpolar molecule
Polar molecule
For a list of molecules see the List of compounds
List of molecules in interstellar space