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- Alternate meaning: Iridium (satellite)
|Name, Symbol, Number||Iridium, Ir, 77|
|Chemical series||transition metals|
|Group, Period, Block||9, 6, d|
|Density, Hardness||22650 kg/m3, 6.5|
|Atomic weight||192.217 amu|
|Atomic radius (calc.)||135 (180) pm|
|Covalent radius||137 pm|
|van der Waals radius||no data|
|Electron configuration||[Xe]44f14 5d7 6s2|
|e- 's per energy level||2, 8, 18, 32, 15, 2|
|Oxidation states (Oxide)||2, 3, 4, 6 (mildly basic)|
|Crystal structure||Cubic face centered|
|State of matter||solid (__)|
|Melting point||2739 K (4471 °F)|
|Boiling point||4701 K (8002 °F)|
|Molar volume||8.52 ×1010-6 m3/mol|
|Heat of vaporization||604 kJ/mol|
|Heat of fusion||26.1 kJ/mol|
|Vapor pressure||1.47 Pa at 2716 K|
|Speed of sound||4825 m/s at 293.15 K|
|Electronegativity||2.20 (Pauling scale)|
|Specific heat capacity||130 J/(kg*K)|
|Electrical conductivity||19.7 106/m ohm|
|Thermal conductivity||147 W/(m*K)|
|1st ionization potential||880 kJ/mol|
|2nd ionization potential||1600 kJ/mol|
|Most stable isotopes|
|SI units & STP are used except where noted.|
A platinum family metal, iridium, is white, resembling platinum, but with a slight yellowish cast. Due to its extreme hardness and brittle properties, iridium is difficult to machine, form, or work. Iridium is the most corrosion-resistant metal known
Iridiuim can not be attacked by any acids or by aqua regia, but it can be attacked by molten salts, such as NaCl and NaCN. The specific gravity of this element is only slightly lower than osmium, which is generally considered to be the heaviest element known. However, calculations of density from the space lattice may produce more reliable data for these elements than actual measurements and give a density of 22650 for iridium versus 22661 for osmium. Definitive selection between the two is therefore not possible at this time. It's just too close to call.
Iridium (Latin iris meaning "rainbow") was discovered in 1803 by Smithson Tennant in London, England along with osmium in the dark colored residue of dissolving crude platinum in aqua regia (a mixture of hydrochloric and nitric acid). This element was named after the Latin word for rainbow because its salts are highly colored.
This metal was to make the standard meter bar in Paris, which is an alloy of 90 % platinum and 10 % iridium. In 1960 the Paris meter bar was replaced as definition of the fundamental unit of length (see krypton).
The KT event, marking the temporal border between the Cretaceous and Tertiary eras of geological time, was identified by a thin stratum of iridium. According to many scientists, such as Luis Alvarez of Berkeley, California, this iridium was of extraterrestrial origin, attributed to a asteroid or a comet thought to have struck near what is now the Yucatan Peninsula.
However, there are others such as Dewey M. McLean of Virginia Polytechnic Institute who argue that the iridium was of volcanic origin. The Earth's core is rich in iridium, and Piton de la Fournaise on Réunion, for example, is still releasing iridium today.
Iridium is found uncombined in nature with platinum and other platinum group metals in alluvial deposits. Naturally occurring iridium alloys include osmiridium and iridiosmium, both of which are mixtures of iridium and osmium. It is recovered commercially as a by-product from nickel mining and processing.
There are two natural isotopes of iridium, and many radioisotopes, the most stable being Ir-192 with a half-life of 73.83 days. Ir-192 decays into platinum, while most of the other radioisotopes decay into osmium.