What is the #"oxidation number"# of nitrogen in #"ammonia, ammonium ion, and nitrate ion"#?
While oxidation number is a formalism, it is a convenient formalism. For ions and molecules, the SUM of the oxidation numbers ALWAYS equals the charge on the ion.
TO illustrate, let's take
ANd thus for ammonia,
And for ammonium,
But for nitrate ion, oxygen is MORE electronegative than nitrogen, and it gets the electrons,