Here's why:

* Caesium (Cs) is in group 1 of the periodic table, meaning it has a +1 charge as a cation (Cs⁺).

* Phosphorus (P) is in group 15 of the periodic table, meaning it has a -3 charge as an anion (P³⁻).

To balance the charges, you need three caesium ions (3 x +1 = +3) to combine with one phosphide ion (-3).