Osmium is a hard heavy metallic element, whose symbol is Os. It has the greatest density of the known chemical elements. It forms octavalent compounds, as OsO4. Osmium is used chiefly as a catalyst, in alloys, and in the manufacture of electric light filaments. It has an atomic weight 190.2, atomic number 76, and valence II, III, IV, and VIII. Its melting point is 3,000º C.