Iridium

Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density of 22.56 g/cm³ (0.815 lb/cu in) as defined by experimental X-ray crystallography. It is one of the most corrosion-resistant metals, even at temperatures as high as 2,000 °C (3,630 °F). However, corrosion-resistance is not quantifiable in absolute terms; although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable, whereas gold dust is not flammable but can be attacked by substances that iridium resists, such as aqua regia.

Iridium, ₇₇Ir

Iridium
Pronunciation	/ɪˈrɪdiəm/ ​(i-RID-ee-əm)
Appearance	silvery white
Standard atomic weight Ar°(Ir)
	192.217±0.002 192.22±0.01 (abridged)
Iridium in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Rh ↑ Ir ↓ Mt osmium ← iridium → platinum
Atomic number (Z)	77
Group	group 9
Period	period 6
Block	d-block
Electron configuration	[Xe] 4f14 5d7 6s2
Electrons per shell	2, 8, 18, 32, 15, 2
Physical properties
Phase at STP	solid
Melting point	2719 K ​(2446 °C, ​4435 °F)
Boiling point	4403 K ​(4130 °C, ​7466 °F)
Density (near r.t.)	22.56 g/cm3
when liquid (at m.p.)	19 g/cm3
Heat of fusion	41.12 kJ/mol
Heat of vaporization	564 kJ/mol
Molar heat capacity	25.10 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 2713 2957 3252 3614 4069 4659
Atomic properties
Oxidation states	−3, −1, 0, +1, +2, +3, +4, +5, +6, +7, +8, +9
Electronegativity	Pauling scale: 2.20
Ionization energies	1st: 880 kJ/mol 2nd: 1600 kJ/mol
Atomic radius	empirical: 136 pm
Covalent radius	141±6 pm
Spectral lines of iridium
Other properties
Natural occurrence	primordial
Crystal structure	​face-centered cubic (fcc)
Speed of sound thin rod	4825 m/s (at 20 °C)
Thermal expansion	6.4 µm/(m⋅K)
Thermal conductivity	147 W/(m⋅K)
Electrical resistivity	47.1 nΩ⋅m (at 20 °C)
Magnetic ordering	paramagnetic
Molar magnetic susceptibility	+25.6 × 10−6 cm3/mol (298 K)
Young's modulus	528 GPa
Shear modulus	210 GPa
Bulk modulus	320 GPa
Poisson ratio	0.26
Mohs hardness	6.5
Vickers hardness	1760–2200 MPa
Brinell hardness	1670 MPa
CAS Number	7439-88-5
History
Discovery and first isolation	Smithson Tennant (1803)
Isotopes of iridium v e
Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct 188Ir synth 1.73 d ε 188Os 189Ir synth 13.2 d ε 189Os 190Ir synth 11.8 d ε 190Os 191Ir 37.3% stable 192Ir synth 73.827 d β− 192Pt ε 192Os 192m2Ir synth 241 y IT 192Ir 193Ir 62.7% stable 193mIr synth 10.5 d IT 193Ir 194Ir synth 19.3 h β− 194Pt 194m2Ir synth 171 d IT 194Ir
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Iridium was discovered in 1803 among insoluble impurities in natural platinum. Smithson Tennant, the primary discoverer, named it after the Greek goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is one of the rarest elements in Earth's crust, with annual production and consumption of only 3 tonnes (6.6 thousand pounds). ¹⁹¹Ir and ¹⁹³Ir are the only two naturally occurring isotopes of iridium, as well as the only stable isotopes; the latter is the more abundant.

The dominant uses of iridium are the metal itself and its alloys, as in high-performance spark plugs, crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the chloralkali process. Important compounds of iridium are chlorides and iodides in industrial catalysis. Iridium is a component of some OLEDs.

Iridium is found in meteorites in much higher abundance than in the Earth's crust. For this reason, the unusually high abundance of iridium in the clay layer at the Cretaceous–Paleogene boundary gave rise to the Alvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 66 million years ago, now known to be produced by the impact that formed the Chicxulub crater. Similarly, an iridium anomaly in core samples from the Pacific Ocean suggested the Eltanin impact of about 2.5 million years ago.

It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum-group metals, the high density and tendency of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.