An aluminium atom has 13 electrons around its nucleus. There are 2 in the K shell, 8 in the L shell and 3 in the M shell.
By giving up the three electrons in its outermost M shell, aluminium becomes a triply charged positive ion, \mathrm{Al}^{3+}.
Aluminium is in group 13 of the periodic table. It is the third most abundant element in the Earth's crust, after oxygen and silicon, and the most abundant of all the metallic elements.
Aluminium combines readily with oxygen. As soon as it meets the air, a thin film of aluminium oxide, \mathrm{Al_2O_3}, forms on its surface, and oxygen can no longer reach the metal underneath. So the corrosion goes no deeper.
Aluminium whose film has been made thicker on purpose by electrolysis is called anodized aluminium, and it is used for pans and kettles.
Even in concentrated nitric acid, this film forms on the surface and the metal stops dissolving. This state is called passivity.
Aluminium reacts both with acids, such as hydrochloric acid, and with strong bases, such as sodium hydroxide solution, giving off hydrogen. The oxide film on the surface has to dissolve first, so it takes a little while before bubbles appear.
2\mathrm{Al} + 6\mathrm{HCl} \longrightarrow 2\mathrm{AlCl_3} + 3\mathrm{H_2}
2\mathrm{Al} + 2\mathrm{NaOH} + 6\mathrm{H_2O} \longrightarrow 2\mathrm{Na[Al(OH)_4]} + 3\mathrm{H_2}
A metal that reacts with both acids and strong bases in this way is called an amphoteric metal. Zinc, tin and lead are amphoteric metals too.
The raw material of aluminium is an ore called bauxite. Pure aluminium oxide, known as alumina, is extracted from the bauxite.
Aluminium oxide does not melt until it is heated above 2000 °C, so it is dissolved in molten cryolite and electrolysed at about 1000 °C. Electrolysing a substance in its molten state like this is called molten salt electrolysis.
At the cathode, aluminium ions gain electrons and become aluminium; at the anode, the carbon electrode combines with oxygen to form carbon dioxide and other gases.
Aluminium forms ions so readily that electrolysing a solution in water only gives off hydrogen, and no aluminium can be obtained. Because the electrolysis uses a huge amount of electricity, making aluminium needs far more energy than making most other metals.
Melting down used aluminium cans and making them into new ones takes only about 3 % of the energy needed to make aluminium from bauxite. For this reason, aluminium cans are collected and made into cans and other products again. In Japan, the recycling rate is more than 90 %.
Japan's 1-yen coin is made of pure aluminium and weighs exactly 1 g.