Here's why:

* Phosphorus (P) is a nonmetal in Group 15 of the periodic table.

* Nonmetals tend to gain electrons to achieve a stable octet (8 electrons) in their outer shell.

* Phosphorus needs to gain three electrons to complete its octet.

* When it gains these electrons, it forms a negatively charged ion with a charge of 3-.

Therefore, the phosphide ion is written as P³⁻.