Here's why:

* Octet Rule: Atoms strive to have a full outer shell of electrons, ideally with 8 electrons (the octet rule).

* Chlorine's Configuration: Chlorine has 7 electrons in its outer shell.

* Gaining an Electron: By gaining one electron, chlorine will have 8 electrons in its outer shell, completing the octet and achieving a stable configuration similar to the noble gas Argon.