Molecular autoionization

In chemistry, molecular autoionization (or self-ionization) is a chemical reaction between molecules of the same substance to produce ions. If a pure liquid partially dissociates into ions, it is said to be self-ionizing. In most cases the oxidation number on all atoms in such a reaction remains unchanged. Such autoionization can be protic (hydron (chemistry)H+ transfer), or non-protic. Protic solvents often undergo some autoionization (in this case autoprotolysis): 2 H2O H3O+ + OH- The self-ionization of water is particularly well studied, due to its implications for acid-base chemistry of aqueous solutions. 2 NH3 NH4+ + NH2- 2 H2SO4 H3SO4+ + HSO4- 3 HF H2F+ + HF2- Here proton transfer between two HF combines with homoassociation of and a third HF to form 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. 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 .