Oxidation state
“Iron(III)” means Fe is assigned +3.
With oxide assigned −2, two Fe(III) centers balance three oxide units in the stoichiometric formula.
Extended solid · iron oxide
Iron(III) oxide
Fe₂O₃
Iron(III) oxide is Fe₂O₃, an extended solid with a 2:3 iron-to-oxygen ratio. The name “iron(III)” records iron in the +3 oxidation state for chemical bookkeeping.
Material distinction: rust is not simply pure Fe₂O₃. Real corrosion products can contain hydrated iron oxides, iron oxyhydroxides and other phases.
Essentials
Fe₂O₃ is a formula unit for an extended solid, not a five-atom molecule.
The formula records the overall 2:3 stoichiometric ratio. Solid iron(III) oxide contains a repeating three-dimensional arrangement rather than isolated Fe₂O₃ molecules.
Detail
Fe³⁺ / O²⁻ notation is useful bookkeeping, but the crystal is an extended bonded solid.
The oxidation-state model explains charge balance cleanly. Real solid-state bonding includes electrostatic and covalent contributions distributed through the crystal.
Crystal form
Hematite is a major crystalline form of Fe₂O₃.
Crystal structure specifies how atoms are arranged; the chemical formula alone does not encode the full lattice.
Understand
Rust is a corrosion material, not a single pure compound formula.
Depending on water, oxygen, salts and time, corrosion can produce several iron oxide and oxyhydroxide phases. Fe₂O₃ is therefore one relevant compound, not a universal molecular formula for rust.
Same element, different oxide
Iron can form more than one oxide.
FeO, Fe₃O₄ and Fe₂O₃ differ in stoichiometry and iron oxidation-state distribution.
Reference identifiers
Names and identifiers that pin down the chemical identity.
| Formula | Fe₂O₃ |
|---|---|
| CAS Registry Number | 1309-37-1 |
| Common crystalline form | hematite |
Advanced
Push past the first model without losing the simple picture.
Is Fe₂O₃ made of Fe₂O₃ molecules?
No. Ordinary solid iron(III) oxide is an extended crystalline solid.
Is every red-brown rust particle hematite?
No. Rust can contain multiple hydrated and oxyhydroxide phases.
Why is the compound called iron(III) oxide?
The Stock numeral III identifies iron's +3 oxidation state in the compound.
Constituent elements