Nonmetallic elements form ions by valence electrons to complete their outer shell.

Non-metals form ions when they gain electrons.

Explanation:

An ion is formed when an atom looses or gains electron.

Metals are the elements which have a tendency to loose electrons and thus they form cations. Example: Sodium will loose 1 electron to form ion.

Non-metals are the elements which have a tendency to gain electrons and thus they form anions. Example: Fluorine will gain 1 electron to form ion.

Hence, non-metals form ions when they gain electrons.