These days 'Alclad', which is also a brand name, is the metal of choice for aviation structures and refers to the structural make-up of the metal. Alclad has a five percent of overall thickness layer of pure ali on each side sandwhiching ali alloy between. The pure ali is a sacrificial layer to encourage corrosion on its surface rather than on the alloy beneath. 2024 T3 is most commonly used in aircraft structures and has other alloying metals, like others, but copper is the highest percent of alloy material. The following numbers show the biggest percentage of alloying material with ali:
1xxx = pure aluminium, 2xxx = copper, 3xxx = manganese, 4xxx = silicon, 5xxx = magnesium, 6xxx = magnesium and silicon, 7xxx = zinc.
Its also worth remembering that aluminium was originally named 'Aluminum', but the '-ium' was added to match it with other elements that had a similar suffix, like strontium, lithium etc...