Aluminum Oxide (Corundum) (Al₂O₃) — Properties, Uses & Safety

Overview

Aluminum oxide (Al2O3), also known as alumina or in its crystalline mineral form as corundum, has a molar mass of 101.96 g/mol. It appears as a white powder in its pure form but is perhaps best known for its colored crystalline varieties: ruby (red, colored by chromium impurities) and sapphire (blue, colored by iron and titanium). Aluminum oxide is an ionic compound with an extremely high melting point of 2,072 °C, reflecting the strong electrostatic attraction between Al3+ and O2− ions in its crystal lattice. It is the second hardest natural mineral after diamond, scoring 9 on the Mohs hardness scale, which makes it invaluable as an abrasive for grinding, polishing, and sandblasting. Aluminum oxide forms naturally as a thin, protective passivation layer (about 4 nm thick) on the surface of aluminum metal when exposed to air, preventing further corrosion — this is why aluminum appears resistant to rusting despite being a reactive metal. This passivation phenomenon is a key concept in materials science and corrosion chemistry. Industrially, aluminum oxide is the primary raw material for producing aluminum metal through the Hall-Héroult electrolysis process, one of the most important electrochemical processes in the world. It is also used as a refractory material in furnace linings, as a catalyst support in chemical engineering, in ceramic applications, and as a substrate for electronic circuits. Aluminum oxide has no significant GHS hazard classifications. For chemistry students, Al2O3 illustrates ionic bonding with high-charge ions, lattice energy and its relationship to melting point, the concept of passivation and corrosion resistance, electrolysis and electrochemistry (the Hall-Héroult process), crystal field theory (explaining the colors of ruby and sapphire), and the Mohs hardness scale. It connects fundamental chemistry to gemology, metallurgy, and materials engineering.

IUPAC Name

aluminum oxide

CAS Number

1344-28-1

PubChem CID

9989226

Molecular Formula

Al₂O₃

Molar Mass

101.961 g/mol

State at STP

solid

Appearance

White powder to crystalline solid; ruby/sapphire are colored forms