Here's why:

* Phosphorus is a nonmetal in group 15 of the periodic table, with a common valency of +3 or +5.

* Bromine is a halogen in group 17, with a valency of -1.

To achieve a neutral compound, one phosphorus atom (with a +3 charge) bonds with three bromine atoms (each with a -1 charge). This results in the chemical formula PBr₃.