* Ionic bonds form between a metal and a nonmetal, or between a polyatomic ion (like ammonium) and a nonmetal.

* NH₄⁺ is the ammonium ion, a positively charged polyatomic ion.

* NO₃⁻ is the nitrate ion, a negatively charged polyatomic ion.

The electrostatic attraction between the positively charged ammonium ion and the negatively charged nitrate ion forms the ionic bond in NH₄NO₃.