Here's why:

* Elements: The periodic table is specifically designed to organize and display the fundamental building blocks of matter – elements. Each element is defined by its unique atomic number (number of protons in its nucleus).

* Compounds: Compounds are formed when two or more elements chemically combine in a fixed ratio. While compounds are made of elements, they are not listed individually on the periodic table.

* Mixtures: Mixtures are combinations of two or more substances (elements or compounds) that are not chemically bonded. Mixtures can be separated by physical means, and they don't have a fixed ratio of components. Mixtures are not represented on the periodic table.