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 (the third shell).

* Octet Rule: Atoms tend to be most stable when they have 8 electrons in their outermost shell (the octet rule).

* Gaining an Electron: Chlorine can achieve a full outer shell by gaining one electron. This would give it an electron configuration of 2-8-8, making it more stable.