Here's why:

* Electron Configuration: Phosphorus has 5 electrons in its outer shell. To achieve a stable octet, it tends to gain 3 electrons.

* Formation of Anions: Gaining 3 electrons results in a phosphorus ion with a -3 charge (P³⁻).

* Common Compounds: Phosphorus forms many compounds with this -3 charge, such as phosphates (PO₄³⁻) found in fertilizers and DNA.

While phosphorus can sometimes have other charges, like +5 in some compounds, -3 is its most common and stable oxidation state.