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Iridium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                77               osmium ← iridium → platinum
                Rh
                ↑
                Ir
                ↓
                Mt

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                      Name, Symbol, Number iridium, Ir, 77
                                         Chemical series transition metals
                                              Group, Period, Block 9, 6, d
                                                  Appearance silvery white
                                             Atomic mass 192.217 (3) g/mol
                               Electron configuration [Xe] 4f^14 5d^7 6s^2
                                   Electrons per shell 2, 8, 18, 32, 15, 2
                                                       Physical properties
                                                               Phase solid
                                      Density (near r.t.) 22.65 g·cm^−3
                                      Liquid density at m.p. 19 g·cm^−3
                                                     Melting point 2719  K
                                                    (2446 ° C, 4435 ° F)
                                                      Boiling point 4701 K
                                                    (4428 ° C, 8002 ° F)
                                         Heat of fusion 41.12 kJ·mol^−1
                                   Heat of vaporization 231.8 kJ·mol^−1
                          Heat capacity (25 °C) 25.10 J·mol^−1·K^−1

   CAPTION: Vapor pressure

                                      P/Pa   1    10  100  1 k  10 k 100 k
                                     at T/K 2713 2957 3252 3614 4069 4659

                                                         Atomic properties
                                     Crystal structure cubic face centered
                                               Oxidation states 2, 3, 4, 6
                                                      (mildly basic oxide)
                                    Electronegativity 2.20 (Pauling scale)
                                       Ionization energies 1st: 880 kJ/mol
                                                          2nd: 1600 kJ/mol
                                                      Atomic radius 135 pm
                                              Atomic radius (calc.) 180 pm
                                                    Covalent radius 137 pm
                                                             Miscellaneous
                                                 Magnetic ordering no data
                               Electrical resistivity (20 °C) 47.1 nΩ·m
                        Thermal conductivity (300 K) 147 W·m^−1·K^−1
                        Thermal expansion (25 °C) 6.4 µm·m^−1·K^−1
                               Speed of sound (thin rod) (20 °C) 4825 m/s
                                                   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
                                                         Selected isotopes

                 CAPTION: Main article: Isotopes of iridium

                               iso     NA   half-life DM DE ( MeV)   DP
                             ^189Ir   syn   13.2 d    ε  0.532     ^189Os
                             ^190Ir   syn   11.8 d    ε  2.000     ^190Os
                             ^191Ir   37.3% Ir is stable with 114 neutrons
                             ^192Ir   syn   73.83 d   β  1.460     ^192Pt
                                                      ε  1.046     ^192Os
                             ^192 mIr syn   241 y     IT 0.155     ^192Ir
                             ^193Ir   62.7% Ir is stable with 116 neutrons
                             ^194Ir   syn   19.3 h    β< 2.247     ^194Pt
                             ^195Ir   syn   2.5 h     β< 1.120     ^195Pt

                                                                References

   Iridium ( IPA: /ɪˈɹɪdiəm/) is a chemical element in the periodic table
   that has the symbol Ir and atomic number 77. A dense, 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 significance in the determination of the probable cause of the
   demise, by a meteorite strike, of the dinosaurs. It is used in high
   temperature apparatus, electrical contacts, and as a hardening agent
   for platinum.

Notable characteristics

   A platinum group 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. Iridium cannot be attacked by
   any acids or by aqua regia, but it can be attacked by molten salts,
   such as NaCl and NaCN.

   The measured density of this element is only slightly lower than that
   of osmium, which is often listed as the most dense 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 kg/m³ for iridium versus 22610 kg/m³ for osmium.
   Definitive selection between the two is therefore not possible at this
   time.

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 compass bearings.
     * Iridium is commonly used in complexes like Ir(mppy)[3] and other
       complexes in polymer LED technology to increase the efficiency from
       25% to almost 100% due to triplet harvesting.
     * Used in high-dose-radiation therapy for the treatment of prostate
       and other forms of cancer
     * Iridium is used in tips of ballpoint pens.
     * Iridium is used as a catalyst for carbonylation of methanol to
       produce acetic acid

   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.

   Iridium was used to tip some early twentieth century fountain pen nibs.
   The tip material in modern pens is still conventionally called
   "iridium," although there is seldom any iridium in it.

History

   Iridium 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). The
   element was named after the Latin word for rainbow (iris; iridium means
   "of rainbows") because many of its salts are strongly colored. Some
   linguists have claimed the word-root is derived from "irid", which
   means "seven" in the Lezghi Language presently spoken in Azerbaijan and
   Daghestan.

   An alloy of 90% platinum and 10% iridium was used in 1889 to construct
   the standard metre bar and kilogram mass, kept by the International
   Bureau of Weights and Measures near Paris. The metre bar was replaced
   as the definition of the fundamental unit of length in 1960 (see
   krypton), but the kilogram prototype is still the international
   standard of mass.

   The KT event of 65 million years ago, marking the temporal border
   between the Cretaceous and Tertiary eras of geological time, was
   identified by a thin stratum of iridium-rich clay. A team led by Luis
   Alvarez ( 1980) proposed an extraterrestrial origin for this iridium,
   attributing it to an asteroid or comet impact near what is now Yucatán
   Peninsula. Their theory is widely accepted to explain the demise of the
   dinosaurs. Dewey M. McLean and others argue that the iridium may have
   been of volcanic origin instead. 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.

   Iridium is rare on Earth, but relatively common in meteorites.

Isotopes

   There are two natural isotopes of iridium, and many radioisotopes, the
   most stable radioisotope being Ir-192 with a half-life of 73.83 days.
   Ir-192 beta decays into platinum-192, while most of the other
   radioisotopes decay into osmium.

Precautions

   Iridium metal is mostly non-toxic due to its relative unreactivity, but
   iridium compounds should be considered highly toxic.
   Retrieved from " http://en.wikipedia.org/wiki/Iridium"
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   with only minor checks and changes (see www.wikipedia.org for details
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