Here's why:

* Compound: A compound is formed when two or more different elements chemically combine in a fixed ratio. Rust, also known as iron oxide, is formed when iron (Fe) reacts with oxygen (O) in the presence of water (H₂O). The chemical formula for the most common type of rust is Fe₂O₃.

* Alloy: An alloy is a mixture of two or more metals, or a metal and a non-metal. Rust is not a mixture, but a new substance with its own unique properties.

Therefore, rust is a compound because it is formed by the chemical reaction of iron and oxygen, resulting in a new substance with a different chemical formula.