Final answer:
The ammonium ion (NH4+) has zero nonbonding electrons because the nitrogen atom uses all of its valence electrons to form bonds with the four hydrogen atoms, accounting for its positive charge.
Step-by-step explanation:
The ammonium ion (NH4+) has a Lewis structure that reflects its ionic nature and the arrangement of electrons around the nitrogen atom. A neutral nitrogen atom has five valence electrons, and these are used to form bonds with the four hydrogen atoms in the ammonium ion. However, the positive charge of the ammonium ion indicates that there are no nonbonding electrons remaining on the nitrogen atom. Each of the hydrogen atoms is bonded to the nitrogen with a single bond, and these four bonds account for all eight of the valence electrons that would normally be associated with nitrogen (after accounting for the positive charge).
Therefore, to correctly depict the electron distribution, we note that the nitrogen atom is surrounded by four bonding pairs of electrons and zero nonbonding electrons, as it has donated its lone pair of electrons to form the additional bond (since NH3 has a lone pair, but NH4+ does not). The tetrahedral molecular structure of the ammonium ion is maintained due to these four bonds, which also satisfies the criteria for a full outer shell around the nitrogen atom, hence no lone pairs are present.