Here's why:

* Location on the Periodic Table: Chlorine is located in Group 17 (also known as the halogens) of the periodic table. All elements in this group are nonmetals.

* Physical Properties: Chlorine is a gas at room temperature, which is a characteristic of nonmetals.

* Chemical Properties: Chlorine is highly reactive and forms anions (negatively charged ions) when it combines with metals. This is typical of nonmetals.