For the equation to be balanced with whole numbers, you need to double the amount of Fe2O3. This gives 6 oxygen molecules and 4 Fe molecules.
So the number of atoms will be Fe = 4 and O = 6.
For the visual Model Key, you will most likely have, 4 separate Fe molecules and 3 oxygen molecules (since oxygen exists as a diatomic - 2 oxygen molecules are attached to each other). For Fe2O3 you will have 2 molecules that go O-Fe-O-Fe-O.
