Molecular autoionization (or self-ionization) is a 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.:163 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:
5 ⇌ PF−
6 + PF+
Molecular autoionization can occur in gases or solids, but occurs most 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.
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