The chemical property that causes rust to form on a nail is oxidation. Oxidation is the process by which an atom or molecule loses electrons to another atom or molecule. In the case of rust, iron atoms in the nail lose electrons to oxygen atoms in the air. This process is accelerated by the presence of water, which helps to break down the iron oxide layer that forms on the surface of the nail. The rust that forms is a hydrated iron oxide, which is a reddish-brown compound.