If you have to write the products of the chemical reaction Fe+O*2( asterisk means subscript),how would you know the charge of Fe in the compound FeO? If the charge is 2+, then the product would be FeO, but if the charge was 3+, then the product would be Fe*2O*3.
Emily, that is a good question, and I don't know how you know without some experience. Fe heated by a burner undoubtedly produces FeO but iron left in the atmosphere probably forms FeO first, then moisture and acidity get into the act, along with oxygen in the atmosphere, and any FeO formed turns to hydrated Fe2O3 which we typically call rust.
