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How can ammonia form ammonium ions and amide ions through self-ionization?

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Difference in molecular structure between ammonia/amines and quaternary ammonium ions — ammonia Ammonia and amines have a slightly flattened trigonal pyramidal shape with a lone pair of electrons above the nitrogen. In quaternary ammonium ions, this area is occupied by a fourth substituent.

Ammonia undergoes a process known as self-ionization, or autoprotolysis, in which ammonia molecules react with one another to form ions. In this equilibrium reaction, one ammonia molecule acts as a proton donor (acid) and another acts as a proton acceptor (base), resulting in the formation of an ammonium ion and an amide ion.1

The chemical equation for this process is:

$2 N H_{3} ⇌ N H_{4}^{+} + N H_{2}^{-}$

In this reaction, the transfer of a hydrogen ion ( $H^{+}$ ) from one ammonia molecule to another produces the ammonium ion ( $N H_{4}^{+}$ ) and the amide ion ( $N H_{2}^{-}$ ). While ammonia acts as a moderately good ionizing solvent, it self-ionizes to a lesser extent than water.1