Here's why:

* Electron configuration: Chlorine has 17 electrons. Its electron configuration is 2-8-7. This means it has 7 electrons in its outermost shell (valence electrons).

* Octet rule: Atoms tend to gain or lose electrons to achieve a stable configuration with 8 electrons in their outermost shell (octet rule).

* Gaining an electron: Chlorine is one electron away from having a full outer shell. Therefore, it readily gains one electron to become stable.

* Negative charge: When chlorine gains an electron, it becomes negatively charged, denoted as Cl⁻.