Iridium is a metallic element of the platinum group with the atomic number 77 , atomic mass 192.2, boiling point at about 44oo degrees celcuis (7952 degrees farinheight), and a melting point of 2450 c (4442 f) the irduims symbol is Ir. Iriduim was discovered by a British chemist by the name Smithson Tennat (1761-1815) in 1804 and received its name from the iridescent nature of some of its compounds. it is an extremely rare metal, ranking sixtyfirst in order of abundance of the elements in the crust of the earth and is found in alluvial deposits alloyed with platinum as platiniridium, or with osmium as osmiridium.
Iridium is a white, brittle , extremely hard metal. its specific gravity is exceeded only by that of osmium, which is a fraction of one percent denser. it is extremely inhert chemically resisting even the action of aqua regia (q.v.) in its chemical compounds it forms divalent and trivalent salts.
It is used chiefly as an alloying material for platinum; the alloy which contains about 10 percent iridium, is far harder than pure platinum. Platinium-iridium alloys containing larger percentages of iridium are used in the making precision instruments, such as: surgical tools. pen points, and standard weights and lenghts.
Iridium has a yellowish cast to it. salts of iridium are highly colored. it is the most corrosion resistant metal known, not attacked by any acid, but is attacked by molten salts. there are two natural isotopes of iridium, and 4 radioisotopes. The most stable being Ir-192 with a half-life of 73.83 days. Ir-192 decays into platium, while the other radioisotopes decays onto osium. Iridium metal is generally non-toxic due to its relative unreactivity but iridium compounds should be considered highly toxic.