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Iridium
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Iridium
Rh
?
Ir
?
Mt
osmium ? iridium ? platinum
Appearance
silvery white
General properties
Name, symbol, number
iridium, Ir, 77
Pronunciation
/?'r?di?m/
i-RID-ee-?m
Element category
transition metal
Group, period, block
9, 6, d
Standard atomic weight
192.217
Electron configuration
4f14 5d7 6s2
2, 8, 18, 32, 15, 2
Physical properties
Phase
solid
Density (near r.t.)
22.56 g·cm-3
Liquid density at m.p.
19 g·cm-3
Melting point
2739 K, 2466 °C, 4471 °F
Boiling point
4701 K, 4428 °C, 8002 °F
Heat of fusion
41.12 kJ·mol-1
Heat of vaporization
563 kJ·mol-1
Molar heat capacity
25.10 J·mol-1·K-1
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
Electronegativity
2.20 (Pauling scale)
Ionization energies
1st: 880 kJ·mol-1
2nd: 1600 kJ·mol-1
Atomic radius
136 pm
Covalent radius
141±6 pm
Miscellanea
Crystal structure
face-centered cubic
Magnetic ordering
paramagnetic
Electrical resistivity
(20 °C) 47.1 nO·m
Thermal conductivity
147 W·m-1·K-1
Thermal expansion
6.4 µm/(m·K)
Speed of sound (thin rod)
(20 °C) 4825 m·s-1
Young's modulus
528 GPa
Shear modulus
210 GPa
Bulk modulus
320 GPa
Poisson ratio
0.26
Mohs hardness
6.5
Vickers hardness
1760 MPa
Brinell hardness
1670 MPa
CAS registry number
7439-88-5
Most stable isotopes
Main article: Isotopes of iridium
iso
NA
half-life
DM
DE (MeV)
DP
188Ir
syn
1.73 d
e
1.64
188Os
189Ir
syn
13.2 d
e
0.532
189Os
190Ir
syn
11.8 d
e
2.000
190Os
191Ir
37.3%
191Ir is stable with 114 neutrons
192Ir
syn
73.827 d
ß–
1.460
192Pt
e
1.046
192Os
192m2Ir
syn
241 y
IT
0.161
192Ir
193Ir
62.7%
193Ir is stable with 116 neutrons
193mIr
syn
10.5 d
IT
0.080
193Ir
194Ir
syn
19.3 h
ß–
2.247
194Pt
194m2Ir
syn
171 d
IT
?
194Ir
v
t
e
· r
Iridium ( /?'r?di?m/ i-RID-ee-?m) is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element (after osmium) and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.Iridium was discovered in 1803 among insoluble impurities in natural platinum. Smithson Tennant, the primary discoverer, named the iridium for the goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes. 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 of the two.The most important iridium compounds in use are the salts and acids it forms with chlorine, though iridium also forms a number of organometallic compounds used in industrial catalysis, and in research. Iridium metal is employed when high corrosion resistance at high temperatures is needed, as in high-end spark plugs, crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the chloralkali process. Iridium radioisotopes are used in some radioisotope thermoelectric generators.Iridium is found in meteorites with an abundance much higher than its average abundance 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 65 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.
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