What happens chemically when metal rusts?

When metal rusts, it undergoes oxidation, a chemical reaction between the metal and oxygen, typically in the presence of water or moisture. For iron, the process involves iron atoms losing electrons to oxygen molecules, forming iron oxides. Water acts as an electrolyte that facilitates electron transfer. The overall reaction produces iron(III) oxide, commonly written as Fe2O3, which is the reddish-brown substance we recognize as rust. The full process involves intermediate steps, including the formation of iron(II) ions and iron(II) hydroxide before further oxidation converts it to iron(III) oxide hydrate. This electrochemical process is essentially a form of corrosion where the metal gradually breaks down as its atoms are converted into oxide compounds.