Aluminium (Al, Z = 13): From Precious Metal to Everyday Material

Role in astrophysics and cosmochemistry

Aluminium is produced in massive stars through nuclear fusion during the advanced stages of their evolution. The radioactive isotope aluminium-26 (\(\,^{26}\mathrm{Al}\)), with a half-life of 717,000 years, is particularly important in astrophysics. Its decay produces characteristic gamma radiation that allows mapping of active star-forming regions in our galaxy. The presence of aluminium-26 in primitive meteorites indicates that the solar system formed in an environment rich in recent supernovae. This isotope also contributed to the internal heating of the early bodies of the solar system, playing a role in their geochemical differentiation.

History of the discovery of aluminium

Although aluminium is the third most abundant element in the Earth's crust, its extraction has long been a major challenge. In 1807, Humphry Davy (1778-1829) identified the existence of a metal in alumina (aluminium oxide) and proposed the name aluminum. In 1825, Hans Christian Ørsted (1777-1851) produced a small amount of impure aluminium by reducing aluminium chloride with a potassium amalgam. In 1827, Friedrich Wöhler (1800-1882) improved the process and obtained aluminium powder. It was Henri Sainte-Claire Deville (1818-1881) who, in 1854, developed the first industrial process using sodium as a reducing agent. Finally, in 1886, Paul Héroult (1863-1914) in France and Charles Martin Hall (1863-1914) in the United States simultaneously discovered the electrolytic process that revolutionized aluminium production and made it accessible to the general public.

N.B.:
Aluminium was once more precious than gold. In the mid-19th century, before the invention of the Hall-Héroult process, aluminium was more expensive than gold and was reserved for luxury items. Napoleon III owned an aluminium table service that he reserved for his most prestigious guests, while others had to make do with gold tableware. The top of the Washington Monument, inaugurated in 1884, was crowned with a 2.8 kg aluminium pyramid, then the largest piece of cast aluminium in the world, a symbol of modernity and technical progress.

Structure and fundamental properties of aluminium

Aluminium (symbol Al, atomic number 13) is a metal from the group 13 elements (formerly group IIIA). Its atom has 13 protons, 13 electrons, and usually 14 neutrons in its only stable isotope (\(\,^{27}\mathrm{Al}\)).
At room temperature, aluminium is a solid, silvery-white, remarkably light metal (density ≈ 2.70 g/cm³), malleable, ductile, and an excellent conductor of electricity and heat. The melting point of aluminium: 933.47 K (660.32 °C). The boiling point: 2,792 K (2,519 °C). Aluminium spontaneously forms a thin layer of aluminium oxide (Al₂O₃) on its surface, which remarkably protects it from corrosion.

Table of aluminium isotopes

Electronic configuration and electron shells of aluminium

N.B.:
Electron shells: How electrons are organized around the nucleus (new window).

Aluminium has 13 electrons distributed over three electron shells. Its full electronic configuration is: 1s² 2s² 2p⁶ 3s² 3p¹, or simplified: [Ne] 3s² 3p¹. This configuration can also be written as: K(2) L(8) M(3).

Detailed structure of aluminium shells

K shell (n=1): contains 2 electrons in the 1s subshell. This inner shell is complete and very stable.
L shell (n=2): contains 8 electrons distributed as 2s² 2p⁶. This shell is also complete, forming a noble gas configuration (neon).
M shell (n=3): contains 3 electrons distributed as 3s² 3p¹. The 3s orbitals are complete, while the 3p orbitals contain only one electron out of 6 possible. This outer shell is therefore very incomplete.

Valence electrons and oxidation states of aluminium

Aluminium has 3 valence electrons (configuration 3s² 3p¹) in group 13 of the periodic table. It exhibits only one stable oxidation state: +3 (Al³⁺ ion).

Stability of the Al³⁺ ion

By losing its 3 electrons, aluminium forms the Al³⁺ ion, which adopts the electronic configuration of neon [Ne], giving it great stability. Unlike other metals, aluminium does not exhibit intermediate oxidation states.

Chemical properties and bonds

This electronic structure allows it to form various bonds: ionic with non-metals, metallic in its pure state, and sometimes covalent. It has a strong affinity for oxygen.

Passivation and corrosion resistance

In air, aluminium spontaneously forms a thin layer of Al₂O₃ (alumina) oxide that protects it from corrosion. This natural passivation explains its durability despite its intrinsic reactivity.

Industrial importance

Aluminium is the most used metal after iron, appreciated for its lightness (2.7 g/cm³), good electrical and thermal conductivity, malleability, and strength. It is essential in aeronautics, packaging, construction, and the electrical industry. It is the 3rd most abundant element in the Earth's crust.

Chemical reactivity of aluminium

Despite its high thermodynamic reactivity, aluminium appears chemically inert due to the protective oxide layer that forms instantly on its surface. This passivation can be artificially enhanced by anodization. Aluminium reacts with acids (releasing hydrogen) and strong bases (forming aluminates). At high temperatures, it can reduce many metal oxides in highly exothermic reactions (thermite reaction). Aluminium mainly forms compounds in the +III oxidation state, notably aluminium oxide (Al₂O₃), aluminium chloride (AlCl₃), and aluminium sulphate (Al₂(SO₄)₃).

Industrial and technological applications of aluminium

• in aeronautical and aerospace construction to lighten structures (fuselages, wings, tanks);
• in the automotive industry to reduce vehicle weight and improve energy efficiency;
• for manufacturing food packaging (cans, trays, aluminium foil);
• in building construction (windows, facades, metal frames);
• as an electrical conductor in high-voltage lines and cables;
• for producing corrosion-resistant kitchen utensils;
• in the manufacture of mirrors and reflectors due to its reflective power;
• as a reducing agent in extractive metallurgy (thermite process);
• in pyrotechnics to produce sparks and intense heat;
• in the production of glass and ceramics (alumina);
• as a catalyst or catalyst support in industrial chemistry;
• for treating drinking water (aluminium sulphate as a coagulant);
• in electronics for heat sinks and casings;
• in medicine in certain vaccine adjuvants and antacids;
• in solar panels and photovoltaic structures;
• for manufacturing sports equipment (bicycles, golf clubs, ski poles);
• as a pigment in paints and cosmetics;
• in recycling, where aluminium can be remelted indefinitely with only 5% of the energy required for initial production.

Economic and environmental importance of aluminium

Aluminium is the most produced non-ferrous metal in the world, with annual production exceeding 65 million tonnes. Its primary production by electrolysis is very energy-intensive, consuming about 15,000 kWh per tonne of aluminium produced. However, aluminium recycling has a remarkable energy balance: remelting recycled aluminium requires only 5% of the energy needed for primary production. About 75% of all aluminium ever produced is still in use today thanks to recycling. This infinite recyclability without loss of quality makes aluminium a key material for the circular economy and ecological transition.

Extraction and production of aluminium

Aluminium is mainly extracted from bauxite, an ore rich in hydrated aluminium oxide. The Bayer process purifies bauxite into alumina (Al₂O₃). Alumina is then reduced to metallic aluminium by the Hall-Héroult process: electrolysis in a bath of molten cryolite at about 960 °C. The main bauxite-producing countries are Australia, Guinea, Brazil, and China. Aluminium production requires a lot of electricity, so it is concentrated in countries with abundant and cheap hydroelectric energy.