Aluminium is a silvery-white, ductile and malleable metal. It belongs to group IIIA of the Periodic Table, and in compounds it is usually found as AlIII. It forms about 8% of the earth's crust and is one of the most reactive of the common metals. Exposure to water, oxygen or other oxidants leads to the formation of a superficial coating of aluminium oxide, which provides the metal with a high resistance to corrosion. Aluminium oxide is soluble in mineral acids and strong alkalis but insoluble in water, whereas aluminium chloride, nitrate and sulfate are water soluble. Aluminium halogenides, hydride and lower aluminium alkyls react violently with water. Aluminium possesses high electrical and thermal conductivity, low density and great resistance to corrosion. It is often alloyed with other metals. Aluminium alloys are light, strong and readily machined into shapes.