Molybdenum is a group ,0 hexavalent metallic element with the symbol Mo and atomic number 42. It has the sixth highest melting point of any element, and for this reason it is used in high-strength steel alloys. A significant use is in stainless steels. In its pure metal form, molybdenum is silvery white and very hard, though it is somewhat more ductile than tungsten. Only tantalum, osmium, rhenium, and tungsten have higher melting points. It also has the lowest heating expansion of any commercially used metal. The chemical uses of molybdenum are extensive but the greatest chemical use is as a catalyst in the removal of sulfur from petroleum.