Here's why:

* Electronegativity: Chlorine is a highly electronegative element, meaning it has a strong tendency to attract electrons.

* Formation of Anions: When chlorine forms ionic compounds with metals, it gains one electron to achieve a stable octet configuration, forming the chloride ion (Cl⁻). In this case, its oxidation state is -1.

However, chlorine can also have other oxidation states, including:

* +1: In compounds like hypochlorous acid (HOCl)

* +3: In compounds like chlorous acid (HClO₂)

* +5: In compounds like chloric acid (HClO₃)

* +7: In compounds like perchloric acid (HClO₄)

The specific oxidation state of chlorine in a compound depends on the nature of the other atoms it is bonded to.