osmium

Osmium is a chemical element with the symbol Os and atomic number 76. It is a hard, blue-gray transition metal and a member of the platinum group. Osmium is notable for its extremely high density, among the highest of all elements, and for forming a range of oxides that are used in specialized applications.

In terms of physical properties, osmium has a density of about 22.59 g/cm3 at room temperature. It has a very high melting point, around 3,033 degrees Celsius, and a boiling point well over 5,000 degrees Celsius. The metal is hard and brittle and crystallizes in a hexagonal close-packed structure. It resists corrosion in many environments, but many of its compounds, especially osmium tetroxide, OsO4, are volatile and highly toxic.

Osmium is relatively rare in the Earth's crust and is typically recovered as a byproduct of refining platinum group metals from platinum ore, and to a lesser extent from nickel and copper ores. The metal is often alloyed with iridium to form osmiridium, an extremely hard material used for applications such as electrical contacts, wear-resistant parts, and historically for fountain pen nibs.

Chemically, osmium forms several oxides, with osmium tetroxide (OsO4) being the most important due to its volatility and distinct odor. OsO4 is a powerful oxidizer and a known toxin, requiring careful handling and containment. The element also features in geochronology: the decay of 187Re to 187Os is used in Re-Os dating to study geological time scales.

The name osmium derives from the Greek osme, meaning smell, in reference to the odor of its volatile oxides.