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 the crust of the earth. The atomic weight of iridium is 192.22. It melts at about 2410° C (about 4370° F) and boils at about 4130° C (about 7466° F), and has a specific gravity of 22.6. Iridium and osmium are considered the most dense elements. 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