Osmium

Osmium

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 family and is the densest naturally occurring element, with a density of 22.59 g/cm3. It is found in nature as an alloy, mostly in platinum ores; its alloys with platinum, iridium, and other platinum group metals are employed in fountain pen tips, electrical contacts, and other applications where extreme durability and hardness are needed.

Occurrence

Osmium is the least abundant stable element in the Earth's crust with an average mass fraction of 0.05 ppb in the continental crust.Osmium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium (osmium rich), and iridiosmium (iridium rich). In the nickel and copper deposits, the platinum group metals occur as sulfides (i.e., (Pt,Pd)S)), tellurides (e.g., PtBiTe), antimonides (e.g., PdSb), and arsenides (e.g., PtAs2); in all these compounds platinum is exchanged by a small amount of iridium and osmium. As with all of the platinum group metals, osmium can be found naturally in alloys with nickel or copper.

Within the Earth's crust, osmium, like iridium, is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below), impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in the Bushveld igneous complex in South Africa, though the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin in Canada are also significant sources of osmium. Smaller reserves can be found in the United States. The alluvial deposits used by pre-Columbian people in the Chocó Department, Colombia are still a source for platinum group metals. The second large alluvial deposit was found in the Ural Mountains, Russia, which is still mined.

Symbol		Os
Atomic Number		76
Atomic Weight		190.23
Oxidation States		+3,+4
Electronegativity, Pauling		2.2
State at RT		solid
Melting Point, K		3327
Boiling Point, K		5300

Appearance and Characteristics

Harmful effects:

Powdered osmium in air forms the pungent, highly toxic osmium tetroxide (OsO4) which can cause lung, skin and eye damage.

Characteristics:

• Osmium is a rare, lustrous, very hard, brittle, bluish-white metal.
• It is the densest of all the elements. (Although osmium’s density is very similar to iridium’s, osmium’s is slightly higher – both measured and calculated.
• Calculated: Osmium 22.587 ± 0.009 g/cm3   &   Iridium: 22.562 ± 0.009 g/cm3 at 20 OC. See data source.)
• Osmium has the highest melting point and the lowest vapor pressure of the platinum group (ruthenium, rhodium, palladium, iridium, and platinum).

Uses of Osmium

• Osmium is principally used alloyed with other metals in the platinum group to produce very hard alloys.
• An alloy of 90% platinum and 10% osmium is used in surgical implants such as pacemakers and replacement heart valves.
• Osmium tetroxide is used in microscopy as a stain for fatty tissue and in fingerprint detection.