Here's why:

* Phosphorus (P) is a nonmetal and forms a +3 cation (P³⁺).

* Bromine (Br) is also a nonmetal and forms a -1 anion (Br⁻).

* To balance the charges, we need three bromide ions (3 x -1 = -3) for every one phosphorus ion (+3).

* This gives us the formula PBr₃, which is the simplest whole-number ratio of atoms in the compound.