Atoms that gain or lose electrons are called

ions

Explanation:

An atom that has gained or lost an electron is called an ion. Ions are charged particles that have lost or added an electron to their outermost shell. Ions are the basis of any chemical reaction. The combination of ions leads to the formation of bonds between atoms and this results in molecules and compounds.

When atoms are in their ground state, they are otherwise neutral and such would not combine chemically. It is difficult to find elements in such form naturally. Atoms that have ionized by losing or gaining electrons would freely combine with one another in order to establish a more stable configuration.