Molecular autoionization is a reaction between molecules of the same substance to produce ions and is created by semipolar bonds. The oxidation number on all atoms in such a reaction remains unchanged. Such autoionization can be protic (H+ transfer), or non-protic. Some examples of each are:
2 H2O H3O+ + OH-
2 NH3 NH4+ + NH2-
2 H2SO4 H3SO4+ + HSO4-
3 HF H2F+ + HF2-
Non-protic solvents:
2 PF5 PF6- + PF4+
2 SO2 SO32- + SO2+
4 AlCl3 3 AlCl4- + Al3+
This type of autoionization can occur in gases or solids, but occurs more readily in liquids. In gases, the molecules are so far apart that ion formation is very unfavorable, and in solids, there is not enough molecular movement. As long as the phase remains constant, autoionization normally increases substantially with increasing pressure.
The self-ionization of water is particularly well studied, due to its implications for acid-base chemistry of aqueous solutions.