What do you call an atom that has gained an electron?

An atom that has gained one or more electrons is called an anion. It is also known as a negative ion. A positive ion, or a cation is one that has lost one or more electrons.

Elements normally have the same number of protons as electrons. Ions tend to be unstable and reactive because the number of electrons and protons is no longer balanced. A negative ion has too many electrons, so it tends to shed the excess. A positive ion has too few, so it tends to attract more electrons to make up the deficit, usually from a nearby negative ion. Most chemical reactions include the temporary ionization of one or more elements and the recombination of ions to form stable products.