I was asked to predict the formula for ammonium phosphate. The explanation has to be based on the theory concerning ionic compounds.

When the formula for an ionic compound has to be written, the theory says that the positive ion has to be listed first.

If 2 or more polyatomic ions occure, the polyatomic ion has to be put in paranthesis.

Phosphate is one of the most common polyatomic ions,   (PO4)^3-,having -3 negative charges and ammonium, NH4^+, is a member of the first group of ions (alkali metals) and it has one positive charge, +1.

The predicted formula for ammonium phosphate is:

(NH4)3 PO4