Here's how it's determined:

* Ammonium ion (NH₄⁺): This is a polyatomic ion with a +1 charge.

* Iodate ion (IO₃⁻): This is also a polyatomic ion with a -1 charge.

To form a neutral compound, the charges must balance. Since the charges are equal and opposite, we need one of each ion to form a neutral compound, resulting in NH₄IO₃.