Although aluminum is the third most abundant element in the Earth's crust, its extraction was long a major challenge. In 1807, Humphry Davy (1778–1829) identified the existence of a metal in alumina (aluminum oxide) and proposed the name aluminum. In 1825, Hans Christian Ørsted (1777–1851) produced a small amount of impure aluminum by reducing aluminum chloride with a potassium amalgam. In 1827, Friedrich Wöhler (1800–1882) improved the process and obtained aluminum 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 aluminum production and made it accessible to the general public.
Aluminum (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, aluminum is a solid, silvery-white, remarkably lightweight metal (density ≈ 2.70 g/cm³), malleable, ductile, and an excellent conductor of electricity and heat. The melting point of aluminum: 933.47 K (660.32 °C). The boiling point: 2,792 K (2,519 °C). Aluminum spontaneously forms a thin layer of aluminum oxide (Al₂O₃) on its surface, which remarkably protects it from corrosion.
| Isotope / Notation | Protons (Z) | Neutrons (N) | Atomic mass (u) | Natural abundance | Half-life / Stability | Decay / Remarks |
|---|---|---|---|---|---|---|
| Aluminum-27 — \(\,^{27}\mathrm{Al}\,\) | 13 | 14 | 26.981539 u | 100% | Stable | The only stable isotope of aluminum; the basis for all its applications. |
| Aluminum-26 — \(\,^{26}\mathrm{Al}\) | 13 | 13 | 25.986892 u | Cosmic trace | 717,000 years | Radioactive β\(^+\) and electron capture yielding \(\,^{26}\mathrm{Mg}\). Produced in stars and by cosmic rays; used to date meteorites. |
| Aluminum-28 — \(\,^{28}\mathrm{Al}\) | 13 | 15 | 27.981910 u | Not natural | 2.245 minutes | Radioactive β\(^-\) decaying into silicon-28. Produced in laboratories. |
| Aluminum-29 — \(\,^{29}\mathrm{Al}\) | 13 | 16 | 28.980445 u | Not natural | 6.56 minutes | Radioactive β\(^-\) yielding silicon-29. Used in nuclear research. |
| Other isotopes — \(\,^{21}\mathrm{Al}\) to \(\,^{43}\mathrm{Al}\) | 13 | 8 — 30 | — (variable) | Not natural | Milliseconds to seconds | Very unstable isotopes produced artificially; research in nuclear physics. |
N.B. :
Electron shells: How electrons organize around the nucleus.
Aluminum has 13 electrons distributed across three electron shells. Its full electronic configuration is: 1s² 2s² 2p⁶ 3s² 3p¹, or simplified as: [Ne] 3s² 3p¹. This configuration can also be written as: K(2) L(8) M(3).
K Shell (n=1): contains 2 electrons in the 1s subshell. This inner shell is complete and highly 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 six possible. This outer shell is therefore very incomplete.
The 3 electrons in the outer shell (3s² 3p¹) are the valence electrons of aluminum. This configuration explains its chemical properties:
By losing its 3 valence electrons, aluminum forms the Al³⁺ ion (oxidation state +3), its unique and systematic oxidation state in all its compounds.
The Al³⁺ ion then adopts an electronic configuration identical to that of neon [Ne], a noble gas, which gives this ion great stability.
Unlike other metals, aluminum does not exhibit stable intermediate oxidation states; only the +3 state is observed in chemistry.
The electronic configuration of aluminum, with 3 electrons in its valence shell, classifies it in group 13 of the periodic table (boron family). This structure gives it characteristic properties: an exclusive tendency to form the Al³⁺ ion by losing all its valence electrons, the ability to form both ionic (with non-metals) and metallic bonds (crystalline structure), and the ability to form covalent compounds in some cases. Aluminum has a strong affinity for oxygen, spontaneously forming a thin layer of aluminum oxide (Al₂O₃) in the air, which protects it from corrosion. This natural passivation explains its remarkable resistance to corrosion despite its intrinsic reactivity. Its industrial importance is significant: aluminum is the most widely used metal after iron, due to its lightness (density of 2.7 g/cm³), good electrical and thermal conductivity, malleability, and corrosion resistance. It is widely used in aeronautics, packaging, construction, and the electrical industry. Aluminum is the third most abundant element in the Earth's crust.
Despite its high thermodynamic reactivity, aluminum appears chemically inert due to the protective oxide layer that forms instantly on its surface. This passivation can be artificially enhanced by anodizing. Aluminum reacts with acids (releasing hydrogen gas) and strong bases (forming aluminates). At high temperatures, it can reduce many metal oxides in highly exothermic reactions (thermite reaction). Aluminum mainly forms compounds in the +III oxidation state, notably aluminum oxide (Al₂O₃), aluminum chloride (AlCl₃), and aluminum sulfate (Al₂(SO₄)₃).
Aluminum is the most produced non-ferrous metal in the world, with annual production exceeding 65 million tons. Its primary production by electrolysis is highly energy-intensive, consuming about 15,000 kWh per ton of aluminum produced. However, aluminum recycling has a remarkable energy balance: remelting recycled aluminum requires only 5% of the energy needed for primary production. About 75% of all aluminum ever produced is still in use today thanks to recycling. This infinite recyclability without loss of quality makes aluminum a key material for the circular economy and ecological transition.
Aluminum is primarily extracted from bauxite, an ore rich in hydrated aluminum oxide. The Bayer process purifies bauxite into alumina (Al₂O₃). Alumina is then reduced to metallic aluminum by the Hall-Héroult process: electrolysis in a molten cryolite bath at about 960 °C. The main bauxite-producing countries are Australia, Guinea, Brazil, and China. Since aluminum production requires a lot of electricity, it is concentrated in countries with abundant and cheap hydroelectric power.
Aluminum is produced in massive stars by nuclear fusion during the advanced stages of their evolution. The radioactive isotope aluminum-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 aluminum-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 first bodies in the solar system, playing a role in their geochemical differentiation.
N.B.:
Aluminum was once more precious than gold. In the mid-19th century, before the invention of the Hall-Héroult process, aluminum was more expensive than gold and was reserved for luxury items. Napoleon III owned an aluminum 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 aluminum pyramid, then the largest cast aluminum piece in the world, a symbol of modernity and technical progress.
