Here's why:

* Rust is iron oxide. The most common form of rust is iron(III) oxide, also known as ferric oxide (Fe₂O₃).

* Iron oxide is a compound. It's formed by the chemical combination of iron (Fe) atoms and oxygen (O) atoms.

* Compounds are made of molecules. A molecule is formed when two or more atoms bond together. In the case of iron(III) oxide, two iron atoms bond with three oxygen atoms to form a molecule.

Therefore, rust is not simply a collection of iron atoms or oxygen atoms. It's a collection of iron oxide molecules.