Here's why:

* Ammonium ion (NH₄⁺): This is a polyatomic cation formed by the reaction of ammonia (NH₃) with a proton (H⁺). The nitrogen atom in ammonia has a lone pair of electrons which can accept a proton, forming a positive charge.

* Acetate ion (CH₃COO⁻): This is a polyatomic anion formed by the loss of a proton from acetic acid (CH₃COOH). The carboxyl group (-COOH) loses a proton, resulting in a negative charge.

These oppositely charged ions attract each other electrostatically, forming an ionic bond.