Aluminium: light metal, protective film, amphoteric chemistry
Aluminium is reactive in principle yet durable in air because its surface rapidly forms a thin oxide layer. Its oxide and hydroxide react with both acids and strong bases.
1. Structure and properties
Aluminium has electron-shell arrangement 2,8,3. It commonly loses three valence electrons to form Al3+. Metallic bonding gives electrical and thermal conductivity, malleability and ductility. Its low density makes it valuable where mass matters, while alloying raises strength.
Fresh aluminium reacts quickly with oxygen: 4Al + 3O2 → 2Al2O3. The compact, adherent Al2O3 film separates the underlying metal from air and water. This passivation differs from porous rust, which can expose fresh iron. Scratched aluminium usually repassivates if oxygen is available.
| Property | Cause or consequence |
|---|---|
| Low density | Light components and transport structures |
| Good conductivity | Electrical and heat-transfer uses |
| Passive oxide film | Useful atmospheric corrosion resistance |
| Easy alloying | Strength can be increased for structural use |
2. Amphoteric oxide and hydroxide
Amphoteric means able to react with both acids and bases. Aluminium oxide reacts with acid: Al2O3 + 6HCl → 2AlCl3 + 3H2O. In strong aqueous base it forms soluble aluminate species: Al2O3 + 2OH− + 3H2O → 2[Al(OH)4]−.