Aluminium is a silvery-white, soft, non-magnetic chemical element with the symbol Al and atomic number 13. It is a ductile metal in the boron group. By mass, aluminium is the most abundant metal in the Earth's crust. The oxides and sulfates are the most useful compounds of aluminium. The abundance of aluminium decreases relative to other elements at greater depths into Earth's mantle and beyond. The chief ore of aluminium is bauxite. Aluminium is remarkable for its low density and its ability to resist corrosion through the phenomenon of passivation.