Iridium

Iridium,  ₇₇Ir

Iridium metal granules in the hands of prof. Martyn Poliakoff
General properties
Name, symbol	Iridium, Ir
Appearance	Silvery shiny metal
Iridium in the periodic table
Rh ↑ Ir ↓ Mt Osmium ← Iridium → Platinum
Atomic number	77
Standard atomic weight (Ar)	192.217(3)
Group, block	, d-block
Period	period 6
Electron configuration	[Xe] 4f14 5d7 6s2
per shell	2, 8, 18, 32, 15, 2
Physical properties
Silvery-white
Phase	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	19 g/cm3
Heat of fusion	41.12 kJ/mol
Heat of	564 kJ/mol
Molar heat capacity	25.1 J/(mol·K)
pressure
Atomic properties
Oxidation states	−3, −1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
Electronegativity	Pauling scale: 2.20
energies	1st: 880 kJ/mol 2nd: 1600 kJ/mol
Atomic radius	empirical: 136 pm
Covalent radius	141±6 pm
Miscellanea
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	4.71·10-8 Ω·m (at 20 °C)
Magnetic ordering	Paramagnetic
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 Registry Number	7439-88-5
Discovery and first isolation	Smithson Tennant (1803)

Iridium is the chemical element with symbol Ir and atomic number 77. It is a transitional group metal, part of the so called platinum group metals. Iridium is one of the rarest elements in the Earth's crust, with annual production and consumption of only three metric tonnes, making it an expensive metal. However, despite the fact that iridium is rarer than platinum and gold, it is somewhat cheaper due to peculiarities of the precious metal market.

Properties

Chemical

Iridium is the most corrosion resistant metal, even at high temperatures. At standard conditions, no chemical compound can attack it in bulk form. Finely divided however, it is more reactive. At high temperatures, molten sodium cyanide and potassium cyanide will attack iridium, as well as oxygen and fluorine.

Iridium tends to form mixed-valence compounds, most notably iridum(IV) chloride (whose name is particularly unusual).

Iridium will form alloys with metals, such as osmium (osmiridium) and iron.

Iridium's organometallic chemistry is extensively studied, as iridium forms very stable bonds with carbon.

Physical

Iridium is a silvery white, very hard metal. It has excellent mechanical properties, which makes it difficult to work with. It's high melting point of 2446 °C makes it difficult to melt and cast, so powdered metallurgy is usually employed. It is the only metal to maintain good mechanical properties in air at temperatures above 1600 °C, making it useful as a high temperature catalyst. It is the second densest element after osmium, with a value of 22.56 g/cm³. Because of this property, most of the elemental iridium has sunk in the Earth crust.

Availability

In nature it usually found together with osmium, usually in meteorites. However due to the low concentration of Ir in meteorites and rarity of intact meteorites in nature, space rocks are not a particularly good source for the amateur chemist.

Iridium can be found in certain high-performance car spark plugs, in the tip of the electric contact. Because it's immune to most corrosive substances, it's recommended to dissolve the welding around the iridium, with a strong acid. It is not much, but being very dense, just a few spark tips can build up a significant quantity.

Lastly, iridium can be bought as bullions and coins, though in recent years its price has risen to 46 $/gram.

Iridium's high melting point and chemical resistance allows it to be used in the creation of high-performance crucibles which, unlike the ceramic ones, are very thin, thus reducing the amount of iridium used. Such crucibles are widely used for single crystal growth.

Preparation

As it sits at the bottom of the metal reactivity series, it can be reduced very easy from it's compounds.

Projects

• Organoiridium compounds
• Single crystal growth in iridium crucibles

Handling

Safety

Being the least reactive metal, iridium is non-toxic. Finely divided iridium powder can be hazardous to handle, as it is an irritant and may ignite in air.

Storage

Bulk iridium does not require any special storage. Powdered iridium should be stored in closed containers.

Disposal

Due to iridium's rarity and price, it's best to recycle it.

References

Relevant Sciencemadness threads

• Iridium (IV) chloride