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 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).
* Full Shell: To achieve a full shell, chlorine needs one more electron to make its valence shell have 8 electrons.
