What is the oxidation number of nitrogen in the ammonium ion, NH4+?

1. Hydrogen here is combined with a nonmetal, so each H is +1. 2. Four hydrogen atoms contribute 4 times +1, which is +4. 3. The sum of oxidation numbers must equal the ion charge, which is +1. 4. So +1 = (+4) + x for the nitrogen value x. 5. Solving gives x = +1 minus 4, that is -3. 6. Forgetting the ion charge and setting the sum to zero would wrongly give -4. 7. The value +5 belongs to nitrogen in nitrate, where oxygen is the partner instead. _Source: OpenStax Chemistry (CC BY 4.0), Ch 4 "Stoichiometry of Chemical Reactions", section 4.2 Classifying Chemical Reactions_