Aluminium – facts and properties
- Symbol
- Al
- Atomic number
- 13
- Atomic mass
- 26.982 u
- Period
- 3
- Group
- 13
- Category
- metal
- Electron configuration
- [Ne] 3s² 3p¹
- Electronegativity
- 1.61
- Atomic radius
- 118 pm
- Valence electrons
- 3
- State at standard conditions
- solid
- Melting point
- 660.29 °C
- Boiling point
- 2519 °C
- Density
- 2.7 g/cm³
- Discovered
- known since antiquity
Aluminium is a light, silvery-white metal with atomic number 13. It is the most abundant metal in the Earth's crust and, after iron, the most widely used metal of all. Chemically it is quite reactive, yet an aluminium window frame lasts for decades, because a microscopically thin layer of oxide seals its surface. Drinks cans, kitchen foil, bicycle frames and large parts of aircraft are made from it.
Position in the periodic table
Occurrence and extraction
Aluminium is the third most abundant element in the Earth's crust, after oxygen and silicon. It never occurs as a metal but is locked up in feldspars, micas and clays. The only economic ore is bauxite, a reddish rock rich in aluminium hydroxides that forms when rocks weather intensely in tropical climates.
Turning bauxite into metal takes two stages. In the Bayer process the ore is digested with hot sodium hydroxide solution: the aluminium dissolves, while iron oxides stay behind as red mud. Aluminium hydroxide is then precipitated from the solution and heated to give pure aluminium oxide, known as alumina. Alumina melts only at a very high temperature, so it is dissolved in molten cryolite instead. In the Hall-Héroult process an electric current reduces it to liquid aluminium at the cathode, while the carbon anodes burn away to carbon dioxide. The process consumes huge amounts of electricity, which is why smelters are often built near hydroelectric power. Aluminium compounds such as alum were known in antiquity and used to fix dyes, but the metal itself was not isolated until Hans Christian Ørsted and Friedrich Wöhler managed it in the nineteenth century. For decades afterwards it cost more than silver.
Properties and reactions
Aluminium has the electron configuration [Ne] 3s² 3p¹ and three valence electrons. It sits in period 3 and group 13. In its compounds it gives up all three outer electrons and has an oxidation state of plus three. Its electronegativity is 1.61.
With a density of 2.7 g/cm³, aluminium weighs only about a third as much as the same volume of steel. It melts at 660.29 °C and boils at 2519 °C. The pure metal is soft and very ductile, so it can be rolled into foil thinner than a human hair. It conducts heat and electricity well and is not magnetic.
Aluminium reacts readily with oxygen: the instant a fresh surface meets air, it forms a dense, tightly bonded layer of aluminium oxide only a few nanometres thick. This layer protects the metal beneath from further corrosion, a behaviour called passivation. Anodising thickens the oxide layer electrochemically and allows it to be dyed in bright colours. Strong acids and, above all, strong alkalis dissolve the protective film, after which aluminium reacts and gives off hydrogen. Because the metal and its hydroxide react with both acids and bases, they are described as amphoteric. Its strong affinity for oxygen drives the thermite reaction, in which aluminium reduces iron oxide to molten iron; railway tracks are welded this way.
Uses in everyday life and technology
Lightness, corrosion resistance and ease of shaping suit aluminium to transport and building. In cars, aluminium body panels, wheels and engine blocks cut weight and therefore fuel use, and the fuselages of most airliners are made largely of aluminium alloys. On buildings you find aluminium window frames, cladding and roofing. Overhead power lines usually consist of aluminium strands around a steel core, because aluminium conducts well for its weight.
At home, aluminium appears as kitchen foil, drinks cans, coffee capsules and saucepans. Its compounds are widespread too: aluminium oxide is the abrasive corundum, and rubies and sapphires are corundum crystals coloured by traces of other metals. Aluminium sulfate helps water treatment works to clump together fine particles so they can be removed.
Recycling is one of aluminium's great strengths. Scrap can be remelted again and again without any loss of quality, and remelting needs only about a twentieth of the energy required to make new metal from bauxite. As a result, a large share of all the aluminium ever produced is still in use.
Molecules containing this element
- Aluminium phosphide AlP
- Aluminium oxide hydroxide AlO(OH)
- Aluminium hydroxide Al(OH)3
Quick question
Which of these elements shows the strongest metallic character?
FAQ
Why doesn't aluminium rust?
Rust is a flaky iron oxide that falls away and exposes fresh iron underneath. Aluminium instead forms a dense, firmly attached oxide film that seals the surface and stops further corrosion. It can still be attacked in strongly acidic or alkaline conditions, or where it touches a less reactive metal in the presence of moisture.
How is aluminium made?
Bauxite is first refined to pure aluminium oxide in the Bayer process. The oxide is then dissolved in molten cryolite and reduced to aluminium metal by electrolysis, which consumes a great deal of electricity.
Why is recycling aluminium worthwhile?
Used aluminium can be melted down again and again without losing quality, using only a small fraction of the energy needed to produce it from ore. It also avoids red mud and electrolysis emissions, so cans and foil belong in the recycling bin.