* Elements are pure substances that cannot be broken down into simpler substances by chemical means. Examples include sodium (Na) and fluorine (F).

* Compounds are formed when two or more elements chemically combine in a fixed ratio. In sodium fluoride, one sodium atom combines with one fluorine atom.

* Mixtures are combinations of two or more substances that are not chemically bonded. They can be separated by physical means.

Since sodium fluoride is formed by the chemical combination of sodium and fluorine, it is a compound.