77
  Ir  
192.217000
Iridium

Name: Iridium
Symbol: Ir
Atomic Number: 77
AtomicWeight: 192.217000
Family: Transition metals
CAS RN: 7439-88-5
Description: A silvery white very hard and brittle metal.
State (25 C): Solid
Oxidation states: +2, +3, +4, +6

Molar Volume: 8.49 cm3/mole
Valence Electrons: 5d76s2

Boiling Point:  4701K, 4428C, 8002F
Melting Point:
2716K, 2443C, 4429F
Electrons Energy Level: 2, 8, 18, 32, 15, 2
Isotopes: 34 + 2 Stable
Heat of Vaporization: 604 kJ/mol
Heat of Fusion: 26.1 kJ/mol
Density: 22.4 g/cm3 @ 300K
Specific Heat: 0.13 J/gK
Atomic Radius: 1.87
Ionic Radius: 0.625
Electronegativity: 2.2 (Pauling); 1.55 (Allrod Rochow)
Vapor Pressure: 1.47 Pa @ 2443C

1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d7 6s2

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 25% nitric acid (HNO3) and 75% hydrochloric acid (HCl).  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 meter bar and kilogram mass, kept by the international Bureau of Weights and Measures near Paris.  This bar was replaced as the definition of the meter in 1960 when the meter was redefined in terms of the orange-red spectral line of Krypton-86, but the kilogram prototype is still the international standard of mass.

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 NaCl and NaCN.

1s2
2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d7
6s2

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.

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.

Applications

The principal use of iridium is as a hardening agent in platinum alloys. Other uses:

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 ballpoint pens is still conventionally called "iridium," although there is seldom any iridium in it.

KT Boundary

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 Yucatan 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 Reunion, or example, is still releasing iridium today.

Isotopes

There are two natural isotopes of iridium, and 34 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.

atom.gif (700 bytes)

Isotope  
Atomic Mass
 
Half-Life
164Ir 163.99220 ~1 ms
165Ir 164.98752 <1 s
166Ir 165.98582 10.5 ms
167Ir 166.981665 35.2 ms
168Ir 167.97988 161 ms
169Ir 168.976295 780 ms
170Ir 169.97497 910 ms
171Ir 170.97163 3.6 seconds
172Ir 171.97046 4.4 seconds
173Ir 172.967502 9.0 seconds
174Ir 173.966861 7.9 seconds
175Ir 174.964113 9 seconds
176Ir 175.963649 8.3 seconds
177Ir 176.961302 30 seconds
178Ir 177.961082 12 seconds
179Ir 178.959122 79 seconds
180Ir 179.959229 1.5 minutes
181Ir 180.957625 4.90 minutes
182Ir 181.958076 15 minutes
183Ir 182.956846 57 minutes
184Ir 183.95748 3.09 hours
185Ir 184.95670 14.4 hours
186Ir 185.957946 16.64 hours
187Ir 186.957363 10.5) hours
188Ir 187.958853 41.5 hours
189Ir 188.958719 13.2 days
190Ir 189.9605460 11.78 days
191Ir 190.9605940 Stable
192Ir 191.9626050 73.827 days
193Ir 192.9629264 Stable
194Ir 193.9650784 19.28 hours
195Ir 194.9659796 2.5 hours
196Ir 195.96840 52 seconds
197Ir 196.969653 5.8 minutes
198Ir 197.97228 8 seconds
199Ir 198.97380 ~20 seconds

Precautions

40px-Skull_and_crossbones.svg.jpg (1420 bytes) Iridium metal is mostly non-toxic due to its relative unreactivity, but iridium compounds should be considered highly toxic.

atom.gif (700 bytes)

Iridium Data
 

Atomic Structure

  • Atomic Radius: 1.87
  • Atomic Volume: 8.54cm3/mol
  • Covalent Radius: 1.27
  • Cross Section (Thermal Neutron Capture) Barns:  425
  • Crystal Structure: Cubic face centered
  • Electron Configuration:
    1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d7 6s2
  • Electrons per Energy Level: 2, 8, 18, 32, 15, 2
  • Ionic Radius: 0.625
  • Filling Orbital: 5d7
  • Number of Electrons (with no charge): 77
  • Number of Neutrons (most common/stable nuclide): 115
  • Number of Protons: 77
  • Oxidation States: 2, 3, 4, 6
  • Valence Electrons: 5d7 6s2

Chemical Properties

  • Electrochemical Equivalent: 1.793 g/amp-hr
  • Electron Work Function: 5.27eV
  • Electronegativity: 2.2 (Pauling); 1.55 (Allrod Rochow)
  • Heat of Fusion: 26.1 kJ/mol
  • Incompatibilities:
  • Ionization Potential
    • First: 9.1
  • Valence Electron Potential (-eV): 92.2

Physical Properties

  • Atomic Mass Average: 192.22
  • Boiling Point: 4701K, 4428C, 8002F
  • Coefficient of Lineal Thermal Expansion/K-1: 6.4E-6
  • Conductivity
    Electrical: 0.197 106/cm
    Thermal: 1.47 W/cmK
  • Density: 22.4 g/cm3 @ 300K
  • Description:
    Heavy brittle white transition metal.
  • Elastic Modulus:
    • Bulk: 371/GPa
    • Rigidity: 209/GPa
    • Youngs: 528/GPa
  • Enthalpy of Atomization: 628 kJ/mole @ 25C
  • Enthalpy of Fusion: 26.4 kJ/mole
  • Enthalpy of Vaporization: 563.6 kJ/mole
  • Flammablity Class:
  • Freezing Point: see melting point
  • Hardness Scale
    • Brinell: 1670 MN m-2
    • Mohs: 6.5
    • Vickers: 1760 MN m-2
  • Heat of Vaporization: 604 kJ/mol
  • Melting Point: 2716K, 2443C, 4429F
  • Molar Volume: 8.49 cm3/mole
  • Optical Reflectivity: 78%
  • Physical State (at 20C & 1atm): Solid
  • Specific Heat: 0.13 J/gK
  • Vapor Pressure: 1.47 Pa @ 2443C

Regulatory / Health

  • CAS Number
    • 7439-88-5
  • RTECS: 1002594IR
  • OSHA Permissible Exposure Limit (PEL)
    • No limits set by OSHA
  • OSHA PEL Vacated 1989
    • No limits set by OSHA
  • NIOSH Recommended Exposure Limit (REL)
    • No limits set by NIOSH
  • Levels In Humans:
    Note: this data represents naturally occuring levels of elements in the typical human, it DOES NOT represent recommended daily allowances.
    • Blood/mg dm-3: n/a
    • Bone/p.p.m: n/a
    • Liver/p.p.m: n/a
    • Muscle/p.p.m: 2E-5
    • Daily Dietary Intake: n/a
    • Total Mass In Avg. 70kg human: n/a

Who / Where / When / How

  • Discoverer: S.Tennant
  • Discovery Location: London England
  • Discovery Year: 1803
  • Name Origin:
    Latin: iris, iridis (rainbow).
  • Abundance:
    • Earth's Crust/p.p.m.: 0.000003
    • Seawater/p.p.m.: N/A
    • Atmosphere/p.p.m.: N/A
    • Sun (Relative to H=1E12): 7.1
  • Sources:
    Found in gravel deposits with platinum and osmiridium ores. Annual world production is around 3 tons. Canada is the primary mining area for iridium.
  • Uses:
    Used with osmium to tip gold pen points, to make crucible and special containers. Used to make alloys used for standard weights and measures, and heat-resistant alloys. Also used incancer irradiation, hypodermic needles, helicopter spark plugs and as hardening agent for platinum.

Ionization Energy (eV): 9.1 eV
Estimated Crustal Abundance: 110-3 milligrams per kilogram
Estimated Oceanic Abundance:
unknown

Transition Metals
Group 3
(IIIB)
4
(IVB)
5
(VB)
6
(VIB)
7
(VIIB)
8
(VIIIB)
9
(VIIIB)
10 (VIIIB) 11
(IB)
12
(IIB)
Period 4 21
Sc
44.95
22
Ti
47.86
23
V
50.94
24
Cr
51.99
25
Mn
54.93
26
Fe
55.84
27
Co
58.93
28
Ni
58.69
29
Cu
63.54
30
Zn
65.39
Period 5 39
Y
88.90
40
Zr
91.22
41
Nb
92.90
42
Mo
95.94
43
Tc
98.00
44
Ru
101.0
45
Rh
102.9
46
Pd
106.4
47
Ag
107.8
48
Cd
112.4
Period 6 57
La
138.9
72
Hf
178.4
73
Ta
180.9
74
W
183.8
75
Re
186.2
76
Os
190.2
77
Ir
192.2
78
Pt
195.0
79
Au
196.9
80
Hg
200.5
Period 7 89
Ac
227.0
104
Rf
261.0
105
Db
262.0
106
Sg
266.0
107
Bh
264.0
108
Hs
269.0
109
Mt
268.0
110
Ds
269.0
111
Rg
272.0
112
Uub
277.0