Iridium
Iridium, a hard, white, metallic element of the platinum family. It is found in mineral deposits with platinum and osmium. Iridium is the heaviest known substance and is the most corrosion-resistant of all the elements. It is insoluble in all acids. Objects that must withstand extremely high temperatures, such as electric furnace windings, aircraft spark plugs, and special crucibles used in glassmaking, are sometimes made of iridium. Iridium is used as a hardening agent in alloys. The International Prototype Kilogram—the world standard for mass—is made of a platinum-iridium alloy. Platinum-iridium alloys are also used for jewelry and in electrical equipment.
Iridium was first isolated by Smithson Tennant, an English chemist, in 1803.
Symbol: Ir. Atomic number: 77. Atomic weight: 192.22. Specific gravity: 22.6. Hardness: 6–6.5. Melting point: 4,370 F. (2,410 C.). Boiling point: 7,466 F. (4,130 C.). Iridium has two stable isotopes, Ir-191 and Ir-193. It belongs to Group VIII of the Periodic Table and has valences of +3 and +4.