Fe2O3, or rust, is iron oxide formed by a redox reaction of iron and oxygen in the presence of water. Iron is the reducing agent, as it gives up electrons, while oxygen is the oxidizing agent gaining electrons. Simply expressed, Fe plus H2O plus O2 yields Fe2O3.
The rusting of iron is an electromagnetic process that begins with the transfer of electrons from iron to oxygen. Iron becomes an ion the presence of moisture. This is the transition of solid Fe to aqueous Fe2+ and two electrons. The electrons then reduce the oxygen that is dissolved in the H2O, as O2 plus four electrons plus four naturally occurring H+ ions yield two H2O molecules. The Fe2+ ions from the iron oxidation then each react with two OH- ions in water to form Fe(OH)2. Following dehydration, Fe(OH)2 produces rust.