Here's why:

* Elements are the simplest pure substances that cannot be broken down into simpler substances by chemical means. Sodium is a pure substance that exists on the periodic table as a single atom.

* Compounds are formed when two or more elements chemically combine in a fixed ratio. For example, sodium chloride (NaCl) is a compound.

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

Since sodium exists as a single type of atom, it is classified as an element.