An elementary substance found as an oxide in the mineral cassiterite, and reduced as a soft white crystalline metal, malleable at ordinary temperatures, but brittle when heated. It is not easily oxidized in the air, and is used chiefly to coat iron to protect it from rusting, in the form of tin foil with mercury to form the reflective surface of mirrors, and in solder, bronze, speculum metal, and other alloys. Its compounds are designated as stannous, or stannic. Symbol Sn (Stannum). Atomic weight 117.4.