Iron is with one kind of atom that combines with oxygen from the air to form rust (iron oxide). Rust contains two types of atoms, which means it is .

A compound

Explanation:

A compound is a substance (pure) that contains two or more different types of atoms chemically combined together. In other words, a compound is formed from the chemical combination of two or more elements.

According to this question, iron (Fe) is an element with one type of atom that combined with oxygen in the atmosphere to form iron oxide known as RUST. This is primarily what happens when a ironic substance is said to RUST. Rust is a combination (chemical) of two different kind of atoms i.e. iron and oxygen, hence, it is a COMPOUND.