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In chemistry, molecular autoionization (or self-ionization) is a
2 H2O <=> H3O+ + OH-
**The 2 NH3 <=> NH4+ + NH2-
*2 H2SO4 <=> H3SO4+ + HSO4-
*3 HF <=> H2F+ + HF2-
**Here proton transfer between two HF combines with 
2 PF5 <=> PF6- + PF4+
*N2O4 <=> NO+ + NO3-
** Here the nitrogen oxidation numbers change from (+4 and +4) to (+3 and +5).
*2 BrF3 <=> BrF2+ + BrF4-
These solvents all possess atoms with odd atomic numbers, either nitrogen or a halogen. Such atoms enable the formation of singly charged, nonradical ions (which must have at least one odd atomic number atom), which are the most favorable autoionization products. Protic solvents, mentioned previously, use hydrogen for this role. Autoionization would be much less favorable in solvents such as sulfur dioxide or carbon dioxide, which have only even atomic number atoms.
chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
between molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
s of the same substance to produce ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s. If a pure liquid partially dissociates into ions, it is said to be self-ionizing. In most cases the oxidation number
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
on all atoms in such a reaction remains unchanged. Such autoionization can be '' protic'' ( transfer), or ''non-protic''.
Examples
Protic solvents
Protic solvent
In chemistry, a protic solvent is a solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group ), a nitrogen (as in an amine group or ), or fluoride (as in hydrogen fluoride). In general terms, any solvent that contains a labile ...
s often undergo some autoionization (in this case autoprotolysis
In chemistry, autoprotolysis is a chemical reaction in which a proton is transferred between two identical molecules, one of which acts as a Brønsted acid, releasing a proton which is accepted by the other molecule acting as a Brønsted bas ...
):
*self-ionization of water
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H2O, deprotonates (loses the nucleus of one of its hydrogen ...
is particularly well studied, due to its implications for acid-base chemistry of aqueous solutions.
*homoassociation In acid–base chemistry, homoassociation (an IUPAC term) is an association between a base and its conjugate acid through a hydrogen bond. The alternate term ''homoconjugation'' also has wide usage, but is ambiguous because it has another meanin ...
of and a third HF to form
Non-protic solvents
*Coordination chemistry
Autoionization is not restricted to neat liquids or solids. Solutions of metal complexes exhibit this property. For example, compounds of the type are unstable with respect to autoionization .Kamata, K.; Suzuki, A.; Nakai, Y.; Nakazawa, H., "Catalytic Hydrosilylation of Alkenes by Iron Complexes Containing Terpyridine Derivatives as Ancillary Ligands", Organometallics 2012, 31, 3825-3828.See also
*Ionization
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecul ...
*Ion association
In chemistry, ion association is a chemical reaction whereby ions of opposite electric charge come together in solution to form a distinct chemical entity. Ion associates are classified, according to the number of ions that associate with each ...
References
Molecular physics {{Chem-stub