Is Ammonium ion (NH4+ ) a molecular ion?
Ammonium ion is not a molecular ion because it is formed by the coordinate covalent bond between ammonia (NH3) and hydrogen ion (H+) .
Due to lone pair electron ,ammonia is an electron rich specie and can easily donate a pair of electron to electron deficient specie (H+ ion).
Why Ammonium ion is not a molecular ion?
In simple words donation of lone pair electron of the central nitrogen of ammonia to hydrogen ion results in the formation of ammonium ion by coordinate covalent bond or dative bond.
Whereas a molecular ion is formed only by the adding or removing electron/s from a molecule .
Since ,ammonium ion (NH4+) is not formed by the gain or lose of electron .Therefore it is not a molecular ion.
NH4+ is a poly-atomic ion.