What happens when a chlorine atom gains an electron its outer energy shell

It becomes an ion, specifically an ANION.

Explanation:

A stable Chlorine atom would be neutral, or it would have no charge. This means that there is an equal number of electrons and protons (17 to be specific).

When a stable atoms gains or loses an electron, it's charge will not be neutral anymore because there would either be more electrons, than protons; or more protons than electrons.

In the case of your problem, there would be more electrons, so when electrons outnumber the protons, then the atom becomes negatively charged. A negatively charged ion is called an ANION.

Neutral                  ANION

   Cl             →           Cl⁻