Here's why:

* Nonmetals have a higher electronegativity than metals. This means they have a stronger attraction for electrons.

* In ionic bonding, a metal atom loses one or more electrons (becoming a positively charged cation) and a nonmetal atom gains one or more electrons (becoming a negatively charged anion).

Examples of nonmetals that form anions:

* Halogens: Fluorine (F-), Chlorine (Cl-), Bromine (Br-), Iodine (I-)

* Oxygen: Oxide (O2-)

* Nitrogen: Nitride (N3-)

* Sulfur: Sulfide (S2-)

* Phosphorus: Phosphide (P3-)

Key takeaway: Nonmetals gain electrons to become anions, forming ionic bonds with metals.