Here's why:

* Chlorine's Atomic Structure: Chlorine has 17 electrons. Its electron configuration is 2-8-7, meaning it has 7 electrons in its outermost shell.

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

* Stability: By gaining one electron, chlorine achieves a full outer shell of 8 electrons, becoming more stable and resembling the noble gas Argon.