Here's why:

* Elements are the simplest pure substances that cannot be broken down into simpler substances by ordinary chemical means. They are found on the periodic table.

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

* Compounds are formed when two or more elements chemically combine in a fixed ratio.

Platinum is a single, pure substance found on the periodic table, making it an element.