Osmium

76

Os

190.23(3)

  • Name: Osmium
  • Symbol: Os
  • Atomic number: 76
  • Atomic weight: 190.23(3)
  • CAS Registry ID: 7440-04-2
  • Group name: Precious metal
  • Period number: 6
  • Block: d-block
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Description

Osmium is a transition metal that has a lustrous silvery blue color in its natural solid state. It forms a tetroxide when exposed to air, and is the densest element known. Osmium has an atomic weight of 190.23, a melting point of 5491 °F, and a boiling point of 9054 °F. Osmium is extremely volatile and it is also very toxic; for these reasons osmium is rarely used alone. Osmium alloys are often used in the tips of fountain pens, electrical contacts, and phonograph needles.

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Isolation

Osmium occurs in nature in the ore iridosmium; this compound is composed of iridium and osmium. Osmium also occurs frequently with platinum. Isolation procedures of osmium are complex.

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The volatility, toxicity, and complex isolation procedures of osmium make small scale laboratory isolations nearly impossible. The process of osmium isolation begins with treating the ore or byproduct that was formed from processing other metals in order to remove silver, palladium, platinum, and gold from the mixture. For this process, the residue is heated with sodium bisulphate until it melts. Water extractions yield a solution that contain rhodium sulfate; insoluble osmium residues are collected and then melted with sodium peroxide. Extraction of this solution in water yields osmium salts, iridium oxides, and ruthenium. Salt and chlorine gas treatments results in the formation of osmium oxides. These oxides are then dissolved through treatment with alcoholic sodium hydroxide to form a sodium hydroxide osmium complex. The complex is then treated with ammonium chloride to precipitate out the osmium chloride oxygen compound. Further evaporation and drying under hydrogen gas yields the pure osmium product.


General
Name Osmium
Symbol Os
Number 76
Chemical series transition metals
Group 8
Period 6
Block d
Appearance silvery, blue cast
Standard atomic weight 190.23(3)  g·mol−1
Electron configuration [Xe] 4f14 5d6 6s2
Electrons per shell 2, 8, 18, 32, 14, 2
Atomic properties
Crystal structure hexagonal
Oxidation states 8, 7, 6, 5, 4, 3, 2, 1, −1, −2 (mildly acidic oxide)
Electronegativity 2.2 (scale Pauling)
Ionization energies 1st: 840 kJ/mol
(more) 2nd: 1600 kJ/mol
(more)
Atomic radius 130 pm
Atomic radius (calc.) 185 pm
Covalent radius 128 pm
Van der Waals radius
Physical properties
Phase solid
Density (near r.t.) 22.61  g·cm-3
Liquid density at m.p. 20  g·cm-3
Melting point 3306 K (3033 °C, 5491 °F)
Boiling point 5285 K (5012 °C, 9054 °F)
Critical point
Heat of fusion 57.85  kJ·mol-1
Heat of vaporization 738  kJ·mol-1
Heat capacity (25 °C) 24.7  J·mol-1·K-1
Miscellaneous
Magnetic ordering
Electrical resistivity (0 °C) 81.2 nΩ·m
Thermal conductivity (300 K) 87.6  W·m−1·K−1
Thermal expansion (25 °C) 5.1  µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 4940 m/s
Young's modulus
Shear modulus 222  GPa
Bulk modulus
Poisson ratio 0.25
Mohs hardness 7.0
Vickers hardness
Brinell hardness 3920  MPa
CAS registry number 7440-04-2

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