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:
The phosphate (PO4) ionic charges should be 3- while the ammonium (NH4) ionic charges should be +(positive charge). Combining these two would form a compound ammonium phosphate. To determine the formula, cross-multiply the ionic charges, so the chemical formula would be (NH4)3 PO4.