# What is the oxidation number of the molecular ion ammonium NH4+ in ammonium chloride NH4Cl?

Well, the oxidation number of nitrogen in $N {H}_{3}$ is $- I I I$. What do you think it is in ammonium ion, $N {H}_{4}^{+}$? Note that I speak of the oxidation number of an element in a compound, not the compound itself.
By definition, the oxidation number is the charge left on the central atom, when each of the bonding pairs of electrons are removed, with the charge assigned to the most electronegative atom. Nitrogen is more electronegative than hydrogen, so when we break a $N - H$ bond we get (formally!) ${N}^{-}$ and ${H}^{+}$.
We could also formulate ammonium chloride as the hydrochloride adduct of ammonia, i.e. $N {H}_{3} \cdot H C l$. For salts of organic amines, say $E {t}_{3} N \cdot H C l$ or $M {e}_{3} N \cdot H B r$, we could (and do) call them triethylamine hydrochloride, or trimethylamine hydrobromide. Of course, in these ammonium salts, all of the hydrogens are equivalent on the quaternized nitrogen, which formally bears a positive charge. I am certainly willing to entertain a follow up question.