How do you know if the reaction Fe + O2 makes FeO or Fe2O3

When equation writing, should you assume that Fe + O2 will be FeO or Fe2O3? Why?

It depends on the oxidation state of the iron. Iron can exist in two oxidation states: +2 or +3. However, Iron (II) oxide is very rare, and iron will almost always form Iron (III) oxide. Rust, which is what commonly forms from the reaction between oxygen and iron, is the hydrated form of Iron (III) oxide. Essentially, unless the problem specifies, you are probably safe to assume iron and oxygen form iron (III) oxide.

O2 has a 2- anion (needs to gain 2 electrons to reach the octet rule)
But theres two oxygens so we need to double that and get 4

and Fe has a 3+ cation (needs to lose 3 electrons to reach the octet rule)

So O2 and Fe has to match to be able to react.
Therefore, you would need to find a common multiple for the numbers. In this case 6

This means you need to has three Oxygens (2+2+2) to get 6 and this is now 3 oxygens (O3)

and Fe needs to be 6 also needs to be 6 so you would add 3+3 to get 6 and this is now Fe2

That is how it would be Fe2O3

I hope this makes sense!

it depends on the Fe charge. Fe is one among many other metals that have multiple charges. so the problem or equation should probably states the charge of Fe, or simply say Fe(II), meaning iron with a +2 charge. so Fe + O2 makes FeO means Fe has a charge of +2, if Fe + O2 makes Fe2O3 means Fe has a charge of +3.