Answer:
It becomes an ion, specifically an ANION.
Step-by-step 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⁻