Iridium

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Atomic symbol: Ir

Atomic number: 77

Atomic weight: 192.22

Atomic volume: 8.54 cm³/mol

Density: 22.42 g/cm³

Period Number: 6

Group number: 9

Group name: Trans. Met.

Element classification: Metal

States

Phase at room temperature: Solid

Melting Point: 2683.2 K

Boiling point: 4823 K

Heat of fusion: 26.10 kJ/mol

Heat of vaporization: 604.0 kJ/mol

Energies

Ionization Energy: 9.1 eV

1st ionization energy: 880 kJ/mole

2nd ionization energy: kJ/mole

3rd ionization energy: kJ/mole

Electronegativity: 2.2

Electron affinity: 151 kJ/mole

Specific heat: 0.130 J/gK

Heat atomization: 665 kJ/mole atoms

Oxidation & Electrons

Shells: 2,8,18,32,15,2

Electron Shell Configuration: [Xe] 4f14 5d7 6s2

Minimum oxidation number: -1

Maximum oxidation number: 6

Minimum common oxidation number: 0

Maximum common oxidation no: 4

Appearance & Characteristics

Structure:: fcc: face-centered cubic

Color: white

Hardness: 6.25 mohs

Toxicity: ?

Characteristics: hard, densest el, inert

Uses: Pt/Ir sparkplugs

Reactions

Reaction with air: none

Reaction with 6M HCl: none

Reaction with 15M HNO3: none

Reaction with 6M NaOH: none

Other Forms

Number of isotopes: 2

Oxide(s): IrO₂

Hydride(s): none

Chloride(s): IrCl₃

Radius

Atomic Radius: 136 pm

Ionic radius (1- ion): pm

Ionic radius (1+ ion): pm

Ionic radius (2- ion): pm

Ionic radius (2+ ion): pm

Ionic radius (3+ ion): 82 pm

Conductivity

Thermal conductivity: 147 J/m-sec-deg

Electrical conductivity: 188.679 1/mohm-cm

Polarizability: 7.6 A^3

Abundance

Source: nickel ores (sulfides)

Relative abundance solar system: -0.180 log

Abundance earth's crust: -3 log

Estimated crustal abundance: 1×10^-3 milligrams per kilogram

Estimated oceanic abundance: No Data Available

History

(L. iris: rainbow) Tennant discovered iridium in 1803 in the residue left when crude platinum is dissolved by aqua regia. The name iridium is appropriate because its salts are highly colored.

Sources

Iridium occurs uncombined in nature with platinum and other metals of this family in alluvial deposits. It is recovered as a by-product from the nickel mining industry.

Properties

Iridium, a metal of the platinum family, is white (similar to platinum) but with a slight yellowish cast. Because iridium is very hard and brittle, it is hard to machine, form, or work.

It is the most corrosion-resistant metal known, and was used in making the standard meter bar of Paris, which is a 90 percent platinum and 10 percent iridium alloy. This meter bar was replaced in 1960 as a fundamental unit of length (see Krypton).

Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by molten salts, such as NaCl and NaCN. The specific gravity of iridium is to osmium's specific gravity. Calculations of the densities of iridium and osmium from the space lattices give values of 22.65 and 22.61 g/cm^3, respectively. These values may be more reliable than actual physical measurements for determining which element is heavier.

Uses

Although its principal use is as a hardening agent for platinum, iridium is also used to make crucibles and devices requiring high temperatures. It is also used for electrical contacts.

The element forms an alloy with osmium which is used for tipping pens and compass bearings.

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