Here's why:

* Element: Chromium is an element, meaning it is composed of only one type of atom.

* Fixed Composition: A pure substance has a fixed and definite composition, which means the ratio of its constituent atoms is always the same.

* Uniform Properties: Pure substances have uniform properties throughout the sample.

While chromium can be found in various forms, like metallic chromium, chromium oxide, and chromium compounds, these are still considered compounds of the element chromium and not mixtures.