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Concept
Osmium
Summary
Osmium () is a chemical element with the symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mostly in platinum ores. Osmium is the densest naturally occurring element. When experimentally measured using X-ray crystallography, it has a density of 22.59g/cm3. Manufacturers use its alloys with platinum, iridium, and other platinum-group metals to make fountain pen nib tipping, electrical contacts, and in other applications that require extreme durability and hardness.
Osmium is among the rarest elements in the Earth's crust, making up only 50 parts per trillion (ppt). It is estimated to be about 0.6 parts per billion in the universe and is therefore the rarest precious metal.
Osmium has a blue-gray tint and is the densest stable element; it is approximately twice as dense as lead and narrowly denser than iridium. Calculations of density from the X-ray diffraction data may produce the most reliable data for these elements, giving a value of 22.587g/cm3 for osmium, slightly denser than the 22.562g/cm3 of iridium; both metals are nearly 23 times as dense as water, and times as dense as gold.
The reflectivity of single crystals of osmium is complex and strongly direction-dependent, with light in the red and near-infrared wavelengths being more strongly absorbed when polarized parallel to the c crystal axis than when polarized perpendicular to the c axis; the c-parallel polarization is also slightly more reflected in the mid-ultraviolet range. Reflectivity reaches a sharp minimum at around 1.5 eV (near-infrared) for the c-parallel polarization and at 2.0 eV (orange) for the c-perpendicular polarization, and peaks for both in the visible spectrum at around 3.0 eV (blue-violet).
Osmium is a hard but brittle metal that remains lustrous even at high temperatures. It has a very low compressibility. Correspondingly, its bulk modulus is extremely high, reported between 395 and 462GPa, which rivals that of diamond (443GPa).
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