When iron reacts with oxygen, it forms iron oxide. This is a common chemical reaction that happens when iron is exposed to air. The most common form of iron oxide is rust, which is a reddish-brown, flaky substance.

Here's a more detailed explanation:

* Chemical Reaction: The reaction between iron (Fe) and oxygen (O2) is a chemical process called oxidation.

* Formation of Iron Oxide: In this reaction, the iron atoms lose electrons and the oxygen atoms gain electrons. This forms iron oxide, which is a compound with the chemical formula Fe2O3.

* Rust: The most common form of iron oxide is rust. It forms when iron reacts with oxygen in the presence of water. Rust is a hydrated form of iron oxide, meaning it also includes water molecules in its structure.

Rusting is a common problem, as it can weaken and damage iron objects. There are many ways to prevent rust, including painting, coating with oil, or using a galvanizing process to coat the iron with a protective layer of zinc.