The chemical element iridium is classed as a transition metal. It was discovered in 1803 by Smithson Tennant.
|Classification:||Iridium is a transition metal|
|Melting point:||2447 oC, 2720 K|
|Boiling point:||4430 oC, 4703 K|
|Neutrons in most abundant isotope:||116|
|Electron configuration:||[Xe] 4f14 5d7 6s2|
|Density @ 20oC:||22.56 g/cm3|
Reactions, Compounds, Radii, Conductivities
|Atomic volume:||8.54 cm3/mol|
|Structure:||fcc: face-centered cubic|
|Specific heat capacity||0.13 J g-1 K-1|
|Heat of fusion||26.10 kJ mol-1|
|Heat of atomization||671 kJ mol-1|
|Heat of vaporization||563 kJ mol-1|
|1st ionization energy||880 kJ mol-1|
|2nd ionization energy||1600 kJ mol-1|
|3rd ionization energy||–|
|Electron affinity||151 kJ mol-1|
|Minimum oxidation number||-1|
|Min. common oxidation no.||-1|
|Maximum oxidation number||6|
|Max. common oxidation no.||4|
|Electronegativity (Pauling Scale)||2.2|
|Polarizability volume||7.6 Å3|
|Reaction with air||none|
|Reaction with 15 M HNO3||none|
|Reaction with 6 M HCl||none|
|Reaction with 6 M NaOH||none|
|Chloride(s)||IrCl2, IrCl3, IrCl4|
|Atomic radius||136 pm|
|Ionic radius (1+ ion)||–|
|Ionic radius (2+ ion)||–|
|Ionic radius (3+ ion)||82 pm|
|Ionic radius (1- ion)||–|
|Ionic radius (2- ion)||–|
|Ionic radius (3- ion)||–|
|Thermal conductivity||147 W m-1 K-1|
|Electrical conductivity||21.3 x 106 S m-1|
|Freezing/Melting point:||2447 oC, 2720 K|
Discovery of Iridium
Iridium was discovered in 1803, by English chemist Smithson Tennant in London.
He found it in the residue left when crude platinum had been dissolved in aqua regia (a mixture of hydrochloric acid and nitric acid).
Smithson Tennant also discovered osmium at the same time.
Iridium’s name comes from the Latin word ‘iris’, meaning rainbow, because many of its salts are highly colored.
Abnormally high amounts of iridium have been found in rocks dating to the K-T boundary between the Cretaceous and Tertiary periods (65 million years ago).
This has led to a widely held view that an iridium-containing comet struck the Earth at that time, leading to the extinction of the dinosaurs and many other forms of life.
The photo below shows the iridium rich layer, which marks the demise of the dinosaurs. Image Ref. (1).
Appearance and Characteristics
Iridium is considered to be of low toxicity.
Iridium in powder form is a known irritant and is a fire hazard.
Iridium is a rare, hard, lustrous, brittle, very dense platinum-like metal.
Chemically it is very unreactive.
It is the most corrosion-resistant metal known and it resists attack by any acid.
Iridium is attacked by molten salts such as sodium chloride (NaCl) and sodium cyanide (NaCN).
Iridium is generally credited with being the second densest element (after osmium) based on measured density, although calculations involving the space lattices of the elements show that iridium is denser.
Uses of Iridium
The main use of iridium is as a hardening agent for platinum alloys.
With osmium, it forms an alloy that is used for tipping pens, and compass bearings.
Iridium is used in making crucibles and other equipment that is used at high temperatures.
It is also used to make heavy-duty electrical contacts.
Iridium was used in making the international standard kilogram, which is an alloy of 90% platinum and 10% iridium.
Radioactive isotopes of iridium are used in radiation therapy for the treatment of cancer.
Abundance and Isotopes
Abundance earth’s crust: 0.4 parts per billion by weight, 0.05 parts per billion by moles
Abundance solar system: 2 parts per billion by weight, 0.01 parts per billion by moles
Cost, pure: $4200 per 100g
Cost, bulk: $2300 per 100g
Source: Iridium is found in natural alloys with platinum and osmium in alluvial deposits. Commercially, iridium is recovered as a by-product from the nickel mining industry.
Isotopes: Iridium has 34 isotopes whose half-lives are known, with mass numbers from 165 to 198. Naturally occurring iridium is a mixture of two isotopes: 191Ir and 193Ir with natural abundances of 37.3% and 62.7% respectively.
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