Aluminium (Al) is a metallic element with atomic number 13. It is a relatively lightweight metal, which makes it ideal for lightweight vehicles such as airplanes.
Native aluminium (aluminium found in pure form in nature) is very rare, and aluminium was therefore extremely expensive in the past. However, aluminium oxide (Al2O3) is plentiful and these days aluminium can be produced by extracting the oxygen from the compound, through a process called electrolysis.
The only stable aluminium isotope is 27Al, with 13 protons and 14 neutrons.
In American English, aluminium is called aluminum, but while we're otherwise using American English on this site, we're using the spelling aluminium since it's the international standard defined by IUPAC.