The equation for the reaction of iron with oxygen to form iron oxide (rust) is:

4 Fe + 3 O₂ → 2 Fe₂O₃

Here's a breakdown:

* Fe: Iron (metal)

* O₂: Oxygen gas

* Fe₂O₃: Iron oxide (rust)

The gas produced in this reaction is oxygen gas (O₂).

This reaction is a combustion reaction and it's an oxidation process.

Important Note: The formation of rust requires the presence of water and is accelerated by the presence of electrolytes (like salt). This means the reaction is more complex in real-world conditions.