Here's why:

* Chlorine is a halogen: Halogens are in group 17 of the periodic table and are highly electronegative. This means they tend to gain electrons to achieve a stable octet.

* Gaining an electron: When chlorine gains an electron, it achieves a -1 charge, thus its oxidation number is -1.

* Examples: In compounds like NaCl (sodium chloride) and HCl (hydrochloric acid), chlorine has an oxidation number of -1.

However, it's important to note that chlorine can exhibit other oxidation numbers depending on the compound it's in. For example:

* +1 in HOCl (hypochlorous acid)

* +3 in HClO2 (chlorous acid)

* +5 in HClO3 (chloric acid)

* +7 in HClO4 (perchloric acid)

* 0 in Cl2 (chlorine gas)