Here's why:

* Location on the Periodic Table: Fluorine is located in Group 17 (VIIA), the halogens, which are all nonmetals.

* Physical Properties: Fluorine exists as a pale yellow gas at room temperature, a characteristic of nonmetals.

* Chemical Properties: Fluorine is highly reactive and readily forms negative ions (anions), another property of nonmetals.

Let me know if you have more questions about elements!