A molecule may be defined as a finite collection of atoms, possibly bearing a net electrical charge, between which there exists some kind of binding. Thus a molecule may be as simple as H2 or or as complex as a polymer chain or a protein chain. The most common kind of chemical binding in molecules is that of the covalent bond, in which the atoms share electrons and thereby lower the overall energy of the molecule relative to that of the separated atoms. Such molecules usually have some degree of stability. Ionic bonding is common in extended arrays of atoms but much less so in small isolated molecules. However, many large molecules have three dimensional conformations which are stabilized in solution by interactions between ionic groups within the molecule. Other kinds of intramolecular forces, such as van der Waals forces and, especially, hydrogen bonding, play an important part in determining the conformation, and hence chemical and biochemical activity, of large molecules. Sometimes two molecules which do not react chemically with each other nevertheless combine or associate by means of hydrogen bonding or van der Waals forces to form a dimer which may itself be considered as a molecule. Such dimers usually dissociate readily.
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