Here's why:

* Element: A pure substance that cannot be broken down into simpler substances by chemical means. Fluorine is a chemical element with the symbol F and atomic number 9.

* Compound: A substance formed when two or more different elements are chemically bonded together.

* Mixture: A combination of two or more substances that are not chemically bonded.

Since fluorine gas is composed of only fluorine atoms bonded together, it fits the definition of an element.