Here's why:

* Chlorine's electron configuration: Chlorine has 7 electrons in its outermost shell.

* Octet rule: To achieve a stable electron configuration, chlorine tends to gain one electron to complete its octet (8 electrons in its outermost shell).

* Ion formation: When chlorine gains an electron, it becomes a negatively charged ion, specifically a chloride ion (Cl⁻). This negative charge signifies a valence of -1.