Periodic Table


Atomic number: 77
Atomic weight: 192.217
Symbol: Ir
Group number: 9
Electronic configuration: [Xe].4f14.5d7.6s2


The name iridium is appropriate, for its salts are highly coloured. Iridium is white, similar to platinum, but with a slight yellowish cast. It is very hard and brittle, making it very hard to machine, form, or work. It is the most corrosion resistant metal known, and was used in making the standard metre bar of Paris, which is a 90% platinum-10% iridium alloy. This metre bar has since been replaced 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.

General information

Discoveror: Smithson Tennant, Antoine Fourcroy, Louis Vanquelin, Hippolyte Collet-Descotils
Date discovered: 1803
Discovered at: England, France
Meaning of name: From the Greek word "iris" meaning "rainbow"

Physical data

Standard state: solid at 298 K
Colour: silvery white
Density of solid at ambient temperature/kg m-3: 22650
Molar volume/cm3: 8.52

Radii /pm

Atomic: 180
Covalent (single bond):
Pauling radius for the ion [Ir]-: no data

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius151.5no data67.820.7


Both values are quoted on the Pauling scale.

Pauling: 2.20
Allred Rochow: 1.55

Crystal Structure

structure: ccp (cubic close-packed)

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 2739
boiling point: 4701

Enthalpies /kJ mol-1

fusion: 26.4
vaporization: 560

single bond enthalpies:
Ir-F Ir-Cl Ir-Br Ir-I Ir-Ir
no data no data no data no data no data

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance
191Ir190.960584 (4)37.3 (5)
193Ir192.962917 (4)62.7 (5)

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/Ir.html