Aluminum (Al), also spelled aluminium, chemical element, a lightweight
silvery white metal of main Group 13 (IIIa, or boron group) of the
periodic table. Aluminum is the most abundant metallic element in Earth’s
crust and the most widely used nonferrous metal. Because of its chemical
activity, aluminum never occurs in the metallic form in nature, but its
compounds are present to a greater or lesser extent in almost all rocks,
vegetation, and animals. Aluminum is concentrated in the outer 16 km (10 miles)
of Earth’s crust, of which it constitutes about 8 percent by weight; it is
exceeded in amount only by oxygen and silicon.