The formula Fe₂O₄ is not a stable compound. It's likely you're referring to Fe₂O₃, which is iron(III) oxide (also known as ferric oxide).

Here's how to calculate the number of atoms:

* Fe₂O₃ tells us there are 2 iron (Fe) atoms and 3 oxygen (O) atoms in one molecule.

* Total atoms: 2 + 3 = 5 atoms

Therefore, there are 5 atoms in one molecule of Fe₂O₃.