<u>Answer:</u> Non-metals form ions when they gain electrons.
<u>Explanation:</u>
An ion is formed when an atom looses or gains electron.
- When an atom looses electrons, it will form a positive ion known as cation.
- When an atom gains electrons, it will form a negative ion known as anion.
Metals are the elements which have a tendency to loose electrons and thus they form cations. <u>Example:</u> 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. <u>Example:</u> Fluorine will gain 1 electron to form ion.
Hence, non-metals form ions when they gain electrons.