Iridium

Iridium, symbol Ir, white, brittle, extremely hard, metallic element. The atomic number of iridium is 77; it is one of the transition elements of the periodic table (see Periodic Law).

Iridium is extremely inert chemically, resisting even the action of aqua regia. In its chemical compounds it forms tetravalent and trivalent salts. It is an extremely rare metal, ranking 77th in order of abundance of the elements in Earth’s crust. The atomic weight of iridium is 192.22. It melts at about 2446°C (about 4435°F) and boils at about 4428°C (about 8002°F). Its specific gravity is 22.6, making iridium more than twice as dense as lead. Iridium and osmium are together considered to be the densest elements; their densities are so close as to be nearly indistinguishable.

Iridium is found in alluvial deposits alloyed with platinum as platiniridium and with osmium as osmiridium. Iridium is used chiefly as an alloying material for platinum; the alloy, which contains about 10 percent iridium, is much harder than pure platinum. Platinum-iridium alloys containing larger percentages of iridium are used in making precision instruments, surgical tools, pen points, and standard weights and lengths.

Iridium was discovered by the British chemist Smithson Tennant in 1804 and was named for the iridescent nature of some of its compounds.