The formula for iron oxide depends on the oxidation state of iron. Here are the two most common forms:

* Iron(II) oxide (ferrous oxide): FeO

* Iron(III) oxide (ferric oxide): Fe₂O₃

There is also a less common form:

* Iron(II, III) oxide (magnetite): Fe₃O₄

It's important to note the Roman numeral in parentheses after "iron" to indicate the oxidation state.