Here's why:

* Zinc (Zn) is a transition metal that typically forms a +2 charge when it becomes an ion.

* Phosphide (P³⁻) is the anion of phosphorus, carrying a -3 charge.

* To form a neutral compound, the charges of the cation (zinc) and anion (phosphide) must balance. The formula for zinc phosphide is Zn₃P₂.

* Three zinc ions (+2 each) have a total charge of +6.

* Two phosphide ions (-3 each) have a total charge of -6.

Therefore, the zinc ion in phosphide has a charge of +2.