Osmium Facts

Chemical & Physical Properties of Osmium

This cluster of osmium crystals was grown using chemical vapor transport.

Osmium Basic Facts

Atomic Number: 76

Symbol: Os

Atomic Weight: 190.23

Discovery: Smithson Tennant 1803 (England), discovered osmium in residue remaining when crude platinum was dissolved in aqua regia

Electron Configuration: [Xe] 4f14 5d6 6s2

Word Origin: from the Greek word osme, a smell or odor

Isotopes: There are seven naturally-occurring isotopes of osmium: Os-184, Os-186, Os-187, Os-188, Os-189, Os-190, and Os-192. Six additional manmade isotopes are known.

Properties: Osmium has a melting point of 3045 +/- 30 °C, boiling point of 5027 +/- 100°C, specific gravity of 22.57, with a valence usually +3, +4, +6, or +8, but sometimes 0, +1, +2, +5, +7. It is a lustrous blue-white metal. It is very hard and remains brittle even at high temperatures. Osmium has the lowest vapor pressure and highest melting point of the platinum group metals. Although solid osmium is unaffected by air at room temperature, the powder will give off osmium tetroxide, a strong oxidizer, highly toxic, with a characteristic odor (hence the metal's name). Osmium is slightly more dense than iridium, so osmium is often credited as being the heaviest element (calculated density ~ 22.61). The calculated density for iridium, based on its space lattice, is 22.65, though the element hasn't been measured as heavier than osmium.

Uses: Osmium tetroxide can be used to stain fatty tissue for microscope slides and to detect fingerprints. Osmium is used to add hardness to alloys. It is also used for fountain pen tips, instrument pivots, and electrical contacts.

Sources: Osmium is found in iridomine and platinum-bearing sands, such as those found in the Americas and Urals. Osmium may also be found in nickel-bearing ores with other platinum metals. Although the metal is difficult to make, the power can be sintered in hydrogen at 2000°C.

Element Classification: Transition Metal

Osmium Physical Data

Density (g/cc): 22.57

Melting Point (K): 3327

Boiling Point (K): 5300

Appearance: blue-white, lustrous, hard metal

Atomic Radius (pm): 135

Atomic Volume (cc/mol): 8.43

Covalent Radius (pm): 126

Ionic Radius: 69 (+6e) 88 (+4e)

Specific Heat (@20°C J/g mol): 0.131

Fusion Heat (kJ/mol): 31.7

Evaporation Heat (kJ/mol): 738

Pauling Negativity Number: 2.2

First Ionizing Energy (kJ/mol): 819.8

Oxidation States: 8, 6, 4, 3, 2, 0, -2

Lattice Structure: Hexagonal

Lattice Constant (Å): 2.740

Lattice C/A Ratio: 1.579

References: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange's Handbook of Chemistry (1952), CRC Handbook of Chemistry & Physics (18th Ed.)

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