76
  Os  
190.230000
Osmium

Name: Osmium
Symbol: Os
Atomic Number: 76
AtomicWeight: 190.230000
Family: Transition Metals
CAS RN: 7440-04-2
Description: A bluish white, extremely hard, and brittle metal, a member of the platinum group.
State (25 C): Solid
Oxidation states: +2, +3, +4, +6, +8

Molar Volume: 8.41 cm3/mole
Valence Electrons: 5d66s2

Boiling Point:  5285K, 5012C, 9054F
Melting Point:
3300K, 3027C, 5481F
Electrons Energy Level: 2, 8, 18, 32, 14, 2
Isotopes: 30 + 6 Stable
Heat of Vaporization: 746 kJ/mol
Heat of Fusion: 31.8 kJ/mol
Density: 22.6 g/cm3 @ 300K
Specific Heat: 0.13 J/gK
Atomic Radius: 1.92
Ionic Radius: 0.63
Electronegativity: 2.2 (Pauling); 1.52 (Allrod Rochow)
Vapor Pressure: 2.52 Pa @ 3027C

1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d6 6s2

History

Osmium (Greek osme meaning "a smell") was discovered in 1803 by Smithson Tennat and William Hyde Wollaston in London, England.

Wollaston and Tennant were looking for a way to purify platinum by dissolution of native platinum ore in aqua regia.  Large amounts of insoluble black powder remained as a byproduct of this operation.

Wollaston concentrated on the soluble portion and discovered Palladium (in 1802) and Rhodium (in 1804), while Tennant examined the insoluble residue.  In the summer of 1803, Tennant identified two new elements; osmium and iridium.  Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.

Characteristics

Osmium in a metallic form is extremely dense, blue white, brittle and lustrous even at high temperatures, but proves to be extremely difficult to make.  Powdered osmium is easier to make, but powdered osmium exposed to air leads to the formation of osmium tetroxide (OsO4), which is toxic.  The oxide is also a powerful oxidizing agent, emits a strong smell and boils at 130C.

1s2
2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d6
6s2

Due to its very high density osmium is generally considered to be the densest known element, narrowly defeating iridium.  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.   If one distinguishes different isotopes, then the highest density ordinary substance would be 192Os.

Osmium also has a very low compressibility.  Correspondingly, its bulk modulus is extremely high -- commonly quoted as 462GPa, which is higher than that of diamond but lower than that of aggregated diamond nanorods - although there is some debate in the academic community about whether it is in fact this high.

This metal has the highest melting point and the lowest vapor pressure of the platinum family.  Common oxidation sates of osmium are +4 and +3, but oxidation states from +1 to +8 are observed.

Occurrence

Turkey with 127,000 tons has the world's largest known reserve of osmium.   Bulgaria also has substantial reserves of about 2500 tons.  This transition metal is also found in iridiosmium, a naturally occurring alloy of iridium and osmium, and in platinum-bearing river sands in the Ural Mountains, and North and South America.   It also occurs in nickel-bearing ores found in the Sudbury, Ontario, Canada region with other platinum group metals.  Even though the quantity of platinum metals found in these ores is small, the large volume of nickel ores processed makes commercial recovery possible.

Value

Osmium is quite valuable, costing about US $400 per troy ounce.  One of the stable isotopes, 187Os, is worth about $25,000 per gram.

Applications

Because of the extreme toxicity of its oxide, osmium is rarely used in its pure state, and is instead often alloyed with other metals that are used in high wear applications.   Osmium alloys such as osmiridium are very hard and, along with other platinum group metals, is almost entirely used in alloys employed in the tips of fountain pens, phonograph needles, instrument pivots, and electrical contacts, as they can resist wear from frequent use.

Osmium tetroxide has been used in fingerprint detection and in staining fatty tissue for microscope slides.  As a strong oxidant, it cross-links lipids by fixing biological membranes in place.  Furthermore, osmium atoms are extremely electron dense, making OsO4 an important stain for transmission electron microscopy (TEM) studies of a wide range of biological materials.  An alloy of 90% platinum and 10% osmium (90/10) is used in surgical implants such as pacemakers and replacement pulmonary valves.

The tetroxide (and a related compound, potassium osmate) are important oxidants for chemical synthesis, despite being very poisonous.

In 1898 an Austrian chemist - Auer von Welsbach - developed the Oslamp with a filament made of osmium, which he introduced commercially in 1902.  After only a few years, osmium was replaced by the more stable metal tungsten (originally known as Wolfram).   Tungsten has the highest melting point of any metal, and using it in light bulbs increases the luminous efficacy and life of incandescent lamps.

Isotopes

Osmium has seven naturally-occurring isotopes, 6 of which are stable: 184Os, 187Os, 188Os, 189Os, 190Os, and (most abundant) 192Os.  The other, 186Os, has an enormously long half-life of 2.0 x 1015 years and for practical purposes can be considered to be stable as well.  187Os is the daughter of 187Re (half-life 4.56 x 1010 years) and is most often measured in an 187Os/188Os ratio.  This ratio, as well as the 187Re/187Os ratio, have been used extensively in dating terrestrial as well as meteoric rocks.  It has also been used to measure the intensity of continental weathering over geologic time and to fix minimum ages for stabilization of the mantle roots of continental cratons.  However, the most notable application of Os in dating has been in conjunction with iridium, to analyze the layer of shocked quartz along the K-T boundary that marks the extinction of the dinosaurs 65 million years ago.

atom.gif (700 bytes)

Isotope  
Atomic Mass
 
Half-Life
162Os 161.98443 1.87 ms
163Os 162.98269 5.5 ms
164Os 163.97804 21 ms
165Os 164.97676 71 ms
166Os 165.972691 216 ms
167Os 166.97155 810 ms
168Os 167.967804 2.06 seconds
169Os 168.967019 3.40 seconds
170Os 169.963577 7.46 seconds
171Os 170.963185 8.3 seconds
172Os 171.960023 19.2 seconds
173Os 172.959808 22.4 seconds
174Os 173.957062 44 seconds
175Os 174.956946 1.4 minutes
176Os 175.95481 3.6 minutes
177Os 176.954965 3.0 minutes
178Os 177.953251 5.0 minutes
179Os 178.953816 6.5 minutes
180Os 179.952379 21.5 minutes
181Os 180.95324 105 minutes
182Os 181.952110 22.10 hours
183Os 182.95313 13.0 hours
184Os 183.9524891 Stable
185Os 184.9540423 93.6 days
186Os 185.9538382 2.0 x 1015 years
187Os 186.9557505 Stable
188Os 187.9558382 Stable
189Os 188.9581475 Stable
190Os 189.9584470 Stable
191Os 190.9609297 15.4 days
192Os 191.9614807 Stable
193Os 192.9641516 30.11 hours
194Os 193.9651821 6.0 years
195Os 194.96813 6.5 minutes
196Os 195.96964 34.9 minutes
197Os   2.8 minutes

Precautions

40px-Skull_and_crossbones.svg.jpg (1420 bytes) Osmium tetroxide is highly toxic.  Airborne low concentrations of osmium can cause lung congestion, skin or eye damage.

atom.gif (700 bytes)

Osmium Data
 

Atomic Structure

  • Atomic Radius: 1.92
  • Atomic Volume: 8.49cm3/mol
  • Covalent Radius: 1.26
  • Cross Section (Thermal Neutron Capture) Barns: 15
  • Crystal Structure: Hexagonal
  • Electron Configuration:
    1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d6 6s2
  • Electrons per Energy Level: 2, 8, 18, 32, 14, 2
  • Ionic Radius: 0.63
  • Filling Orbital: 5d6
  • Number of Electrons (with no charge): 76
  • Number of Neutrons (most common/stable nuclide): 114
  • Number of Protons: 76
  • Oxidation States: 2, 3, 4, 6, 8
  • Valence Electrons: 5d6 6s2

Chemical Properties

  • Electrochemical Equivalent: 1.774 g/amp-hr
  • Electron Work Function: 4.83eV
  • Electronegativity: 2.2 (Pauling); 1.52 (Allrod Rochow)
  • Heat of Fusion: 31.8 kJ/mol
  • Incompatibilities:
  • Ionization Potential
    • First: 8.7
  • Valence Electron Potential (-eV): 91.4

Physical Properties

  • Atomic Mass Average: 190.2
  • Boiling Point: 5285K, 5012C, 9054F
  • Coefficient of Lineal Thermal Expansion/K-1: N/A
  • Conductivity
    Electrical: 0.109 106/cm
    Thermal: 0.876 W/cmK
  • Density: 22.6 g/cm3 @ 300K
  • Description:
    Fine black powder or hard blue-gray mass with a pungent odor.
  • Elastic Modulus:
    • Bulk: 373/GPa
    • Rigidity: 223/GPa
    • Youngs: 559/GPa
  • Enthalpy of Atomization: 669 kJ/mole @ 25C
  • Enthalpy of Fusion: 29.3 kJ/mole
  • Enthalpy of Vaporization: 627.6 kJ/mole
  • Flammablity Class:
  • Freezing Point: see melting point
  • Hardness Scale
    • Brinell: 3920 MN m-2
    • Mohs: 7
  • Heat of Vaporization: 746 kJ/mol
  • Melting Point: 3300K, 3027C, 5481F
  • Molar Volume: 8.41 cm3/mole
  • Physical State (at 20C & 1atm): Solid
  • Specific Heat: 0.13 J/gK
  • Vapor Pressure: 2.52 Pa @ 3027C

Regulatory / Health

  • CAS Number
    • 7440-04-2
  • 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: n/a
    • Daily Dietary Intake: n/a
    • Total Mass In Avg. 70kg human: n/a
  • Discovery Year: 1803
  • Name Origin:
    Greek: osm (odor).
  • Abundance:
    • Earth's Crust/p.p.m.: 0.0001
    • Seawater/p.p.m.: N/A
    • Atmosphere/p.p.m.: N/A
    • Sun (Relative to H=1E12): 5
  • Sources:
    Produced as a by-product of nickel refining. Annual world wide production is around 0.06 tons.
  • Uses:
    Used to tip gold pen points, instrument pivots (such as compass needles and clock bearings), to make electric light filaments. Used for high temperature alloys and pressure bearings.

Ionization Energy (eV): 8.7 eV
Estimated Crustal Abundance: 1.510-3 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