The diffrience is one iron atom and one oxygen atom. Rust is Fe2O3 and black iron oxide Fe3O4.
Black iron oxide is what they use as the recording media.
Like Oxygen and Ozone oxygen is O2 and ozone is O3. Ozone is toxic to humans and oxygen is essential to life.
Also SO2 and SO3, SO2 is a toxic gas, and SO3 is a volitle solid that reacts with water. Hydrogen peroxide and water is a diffrience of one oxygen atom. Bacteria thrive in water but die in Hydrogen peroxide, hence its use as an antiseptic.
Sodium chloride-NaCl sodium is harmful and chlorine is deadly but combined it forms salt, a nessicary nutrient to the body.
And others are toxic either way like lead and mercury.
Point is one atom can signifigantly change the properties of the compound. But I still don't get why water is a liquid and not a gas, logically it should be a gas, but that is off subject.