Iridium

Alternate meaning: Iridium (satellite)

General
Name, Symbol, Number	Iridium, Ir, 77
Chemical series	transition metals
Group, Period, Block	9[?], 6, d
Density, Hardness	22650 kg/m3, 6.5
Appearance	silvery white
Atomic Properties
Atomic weight	192.217 amu
Atomic radius (calc.)	135 (180) pm
Covalent radius	137 pm
van der Waals radius	no data
Electron configuration	[Xe]4f14 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
Physical Properties
State of matter	solid (__)
Melting point	2739 K (4471 °F)
Boiling point	4701 K (1962 °F)
Molar volume	8.52 ×10-3 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
Miscellaneous
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
iso NA half-life DM DE MeV DP 191Ir 37.3% Ir is stable with 114 neutrons Meta {syn.} 241 y IT[?] 0.155 193Ir 62.7% Ir is stable with 116 neutrons
SI units & STP are used except where noted.

Iridium is a chemical element in the periodic table that has the symbol Ir and atomic number 77. A heavy, very hard, brittle, silvery-white transition metal of the platinum family[?], iridium is used in high strength alloys that can withstand high temperatures and occurs in natural alloys with platinum or osmium. Iridium is notable for being the most corrosion resistant element known and for its association with the demise of the dinosaurs. It is used in high temperature apparatus, electrical contacts, and as a hardening agent for platinum.

Table of contents 1 Notable Characteristics 2 Applications 3 History 4 Occurrence 5 Isotopes 6 Precautions 7 External Links

Notable Characteristics 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. Applications The principal use of iridium is as a hardening agent in platinum alloys. Other uses;

• For making crucibles and devices that require high temperatures.
• Electrical contacts (notable example; Pt/Ir sparkplugs).
• Osmium/iridium alloys are used for ball point pen tips and for compass bearings.

At one time iridium, as an alloy with platinum, was used in bushing the vents of heavy ordnance[?] and in a finely powdered condition (iridium black), for painting porcelain black. History 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 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. Occurrence 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. Isotopes 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. Precautions Iridium metal is generally non-toxic due to its relative unreactivity, but iridium compounds should be considered highly toxic.

External Links

• WebElements.com - Iridium (http://www.webelements.com/webelements/elements/text/Ir/index)
• EnvironmentalChemistry.com - Iridium (http://environmentalchemistry.com/yogi/periodic/Ir)