Charge balance
Two Al(III) contributions balance three oxide contributions.
2(+3) + 3(−2) = 0 in oxidation-state bookkeeping.
Extended solid · amphoteric oxide
Aluminium oxide
Al₂O₃
Aluminium oxide is Al₂O₃, an extended aluminium–oxygen solid. The 2:3 formula ratio follows naturally from aluminium in oxidation state +3 and oxide in −2 bookkeeping.
Material distinction: corundum is a crystalline Al₂O₃ structure. Sapphire and ruby are corundum materials whose colors arise from trace substitutions; they are not new bulk formulas replacing Al₂O₃.
Essentials
Al₂O₃ is a formula unit for an extended solid.
The formula records a 2:3 aluminium-to-oxygen ratio. It should not be interpreted as a tiny five-atom Al₂O₃ molecule in ordinary bulk alumina.
Detail
The crystal is an extended bonded network, not free Al³⁺ and O²⁻ particles.
Writing Al³⁺ and O²⁻ is useful for charge and oxidation-state reasoning. The actual solid has extended interactions and significant polarization/covalent character.
Corundum
Crystal structure is additional information beyond formula.
Al₂O₃ can be discussed through specific polymorphs; corundum is the thermodynamically stable form under ordinary ambient conditions.
Understand
Aluminium oxide is amphoteric.
It can participate in acid–base chemistry on both sides: acids can protonate oxide-rich sites, while strong bases can form aluminate species under suitable conditions.
Protective oxide
A thin Al₂O₃-rich surface film helps passivate aluminium metal.
The compact oxide layer slows further reaction by separating the metal from its environment.
Reference identifiers
Names and identifiers that pin down the chemical identity.
| Formula | Al₂O₃ |
|---|---|
| CAS Registry Number | 1344-28-1 |
| Important crystalline form | corundum |
Advanced
Push past the first model without losing the simple picture.
Is Al₂O₃ an ionic compound?
Ionic charge bookkeeping is useful, but real alumina has extended bonding with both ionic and covalent character.
Are ruby and sapphire different compounds from aluminium oxide?
Their corundum host is Al₂O₃; trace substituent ions produce characteristic colors.
What does amphoteric mean here?
Aluminium oxide can react in both acidic and strongly basic chemical environments.
Constituent elements