Atomic number 13

Aluminium

Al
+ phase
− phase

Shells Orbitals 3s 3p

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13 electrons in 3 shells: 2, 8, 3.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Post-transition metal
Period: 3
Also spelled: Aluminum (US)
Group: 13
Block: p
Standard atomic weight: 26.982(1) Da
Electron configuration: [Ne] 3s² 3p¹
Phase at 25 °C: solid
Melting point: 660.32 °C
Boiling point: 2470 °C
Density: 2.70 g/cm³
Electronegativity (Pauling): 1.61
Discovered: 1825, Hans Christian Ørsted, impure metal; Friedrich Wöhler, 1827

Neighbours in the table

Mg Si

Three shells hold aluminium’s thirteen electrons, 2, 8 and 3, and the outer three are a pair in the 3s subshell and one electron in 3p. Take all three away and what is left is an Al³⁺ ion of radius 57 pm. That small radius, with the +3 charge, is what PubChem’s property record credits for the stability of aluminium compounds.

Aluminium’s standard electrode potential in water, measured against the standard hydrogen electrode, is −1.66 V for the Al³⁺/Al pair. The negative sign says the metal parts with those electrons readily enough to reduce the ions of many heavy metals back to metal. Dilute hydrochloric and sulfuric acids take them, and so do sodium and potassium hydroxide, with hydrogen given off in each case. Its Pauling electronegativity of 1.61 measures something different: how tightly an atom holds the electrons it shares in a bond.

The name is older than the metal. It comes from the Latin alumen, the alum that Greeks and Romans used to fix dyes, and Davy proposed aluminium for the still-undiscovered metal in 1807. American publications used that spelling too, and in 1925 the American Chemical Society adopted aluminum. Hans Christian Ørsted made the first impure metal in 1825, and Friedrich Wöhler isolated the pure element two years later.

Where it is found

Aluminium is the commonest metal in the Earth’s crust, and almost none of it is there as the free metal. PubChem puts its share at 8.23 × 10⁴ milligrams per kilogram, or 8.23 per cent of the crust by mass; Los Alamos gives 8.1 per cent. The element is nearly always bound to others, oxygen and silicon and fluorine among them. Native aluminium is a catalogued mineral, IMA1980-085a on the International Mineralogical Association’s list; Los Alamos says the element is never found free in nature. Seawater holds two thousandths of a milligram of it per litre.

The ore that feeds the industry is bauxite, an impure aluminium oxide hydroxide, and clay carries the element as well. Two-fifths to three-fifths of bauxite’s mass is alumina, the common name for aluminium oxide, Al₂O₃. The other of the two commonest compounds is alum itself, potassium aluminium sulfate, the substance the element was named after. The first electrolysis cells dissolved their alumina in cryolite, Na₃AlF₆, mined in Greenland and now replaced by a manufactured mixture of sodium, aluminium and calcium fluorides.

Why it matters

Aluminium ions are among the hardest to reduce, so the metal is prepared by electrolysis. Until that route arrived in 1886 it was a luxury. Henri Sainte-Claire Deville’s method of 1854 made commercial production possible, but the metal stayed too expensive to be widely used. Charles M. Hall took up the problem as a student at Oberlin College in Ohio and electrolysed alumina dissolved in cryolite in 1886. Paul Héroult found the same route in France a month or two later.

Karl Joseph Bayer supplied the other half of the route in 1888, a cheap way to pull alumina out of bauxite. Every tonne of new aluminium still comes out of a Hall-Héroult cell. The Pittsburgh Reduction Company, now Alcoa, made about 25 kilograms a day when it opened in 1888 and about 41,000 by 1909.

Pure aluminium is soft, so the metal in aircraft and rockets carries small additions of manganese, silicon, magnesium or copper. Overhead transmission lines use aluminium though its conductivity is only about 60 per cent of copper’s, because light weight matters more. In 2024 transport took 36 per cent of United States consumption, packaging 23 per cent and building 14 per cent, on the US Geological Survey’s count.

Properties

Properties of aluminium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	13
Standard atomic weight	26.982(1) Da	Da	IUPAC/CIAAW 2024
Melting point	660.32 °C	°C	Los Alamos: 660.32 °C
Boiling point	2470 °C	°C	Los Alamos: 2470 °C
Density	2.70 g/cm³	g/cm³
Electronegativity (Pauling)	1.61
Electron configuration	1s² 2s² 2p⁶ 3s² 3p¹
Phase at 25 °C	solid

Safety

The hazards recorded for aluminium belong to its powder, dust, fume and molten metal rather than to a solid object. Finely divided aluminium ignites easily, which is why the element’s record carries the signal word Danger. The people it concerns are workers who breathe dust or fume over years, since repeated inhalation may affect the lungs and the nervous system. The PubChem Laboratory Chemical Safety Summary collects the NIOSH and ICSC entries and records no acute effects described in the general population.

Questions

Is it aluminium or aluminum?

Both spellings name element 13. Davy proposed the name aluminium in 1807, and that is the spelling IUPAC uses. American publications printed aluminium as well, and in 1925 the American Chemical Society decided to write aluminum in its own journals. The data panel on this page carries both, and the metal is the same either way.

Why doesn't aluminium rust?

It corrodes immediately, and that is what saves it. In moist air a film of oxide grows on the fresh metal and shields what lies underneath, so the attack stops at the surface instead of eating inward. A polished aluminium surface keeps its shine in dry air for the same reason, and evaporated aluminium coatings on telescope mirrors do not deteriorate the way silver coatings do.

Why is aluminium used for drink cans and foil?

It is light and it deforms without tearing. Los Alamos ranks aluminium second among metals for malleability and sixth for ductility, and a cubic centimetre of it weighs 2.7 grams. Beverage cans, pots and pans, house siding, roofing and kitchen foil all use that combination, usually with small amounts of other metals mixed in to make the alloy harder.

Does aluminium cause Alzheimer's disease?

The idea has been tested and is not supported. PubChem's toxicity summary reports that the epidemiological evidence to date does not link Alzheimer's disease to aluminium in drinking water. Most people meet the element through food and water; the foods highest in it include tea, pickled vegetables, pancake mixes, processed cheese, baking powder and spices. PubChem's safety summary is the place to check.

Compounds containing aluminium

Al₂O₃ Aluminium oxide 5 atoms in one formula unit of an ionic lattice.