OSMIUM

Name: Osmium (from the Greek “osme”, literally “smell”)

Symbol: Os

Group: 8

Period: 6

Block: d

Category: Transition metals

Atomic number: 76

Atomic mass: 190.23 u

Electrons per shell: 2, 8, 18, 32, 14, 2

Electronegativity: 2.2

Density: 22.59 g/cc

Melting point: 3033ºC

Boiling point: 5012ºC

Thermal conductivity: 87 W (m·K)

Electrical conductivity: 1.2 × 10^7 S/m

Magnetic order: Paramagnetic

Ordinary state: Solid

Oxidation states: +4

Hardness Mohs: 7

Vickers Hardness: 4137 MPa

Brinell hardness: 3920 MPa

Most stable isotopes: Os-184 (0.02%), Os-186 (1.59%), Os-187 (1.96%), Os-188 (13.24%), Os-189 (16.15%), Os-190 (26.26%) and Os-192 (40.78%)

Discoverer: Smithson Tennant, English (1803)

At the beginning of the 19th century, the search for new platinum-related elements marked a milestone in chemistry, especially in England, where scientists Smithson Tennant and William Hyde Wollaston led the discovery of most of the platinum group metals (PGM), as they are known today. These metals, which include osmium, iridium, rhodium, palladium, and ruthenium, are found associated with platinum in nature. Although platinum, despite its scarcity, seems abundant compared to these elements, the extraction of osmium and iridium is particularly limited, with annual productions rarely exceeding one hundred gross tons. These metals are often obtained as valuable, but costly, byproducts of nickel and copper mining, frequently in the form of native alloys such as osmiridium, a combination of osmium and iridium, or complex mixtures with other platinum group metals, formed in the absence of oxygen, sulfur, or carbon due to their chemically noble nature.

Osmium was identified by Tennant from impure platinum samples, which stood out for their greater rigidity and higher melting point compared to pure platinum. A key characteristic was its insolubility in aqua regia, a corrosive mixture capable of dissolving platinum and palladium, but not osmium, iridium, or rhodium. Using aqua regia to separate the components of these samples, Tennant systematically isolated osmium, though he initially obtained it in the form of an oxide, specifically osmium tetroxide (OsO₄). This compound, known for its toxicity and strong, unpleasant odor, provided clues about the presence of the new element. It was precisely this distinctive aroma that inspired the name "osmium," derived from the Greek "osme" (smell). Once reduced to its pure metallic form, Tennant sent the osmium, along with a note detailing his discovery, to the Royal English Academy of Sciences, ahead of the French chemist Nicholas Vauquelin, who, limited by the scarcity of raw material, could not compete in the race for the discovery.

The history of osmium highlights not only its rarity and unique properties but also the scientific fervor of an era dedicated to unraveling the secrets of noble metals, laying the foundation for its use in modern high-tech applications.

Osmium stands out as one of the most unique metals in the periodic table, thanks to properties that make it unparalleled in the realm of metallurgy. Its color, a distinctive metallic blue hue, differentiates it from other metals like zinc, cadmium, lead, or even its close relatives iridium and rhenium. This bluish tint, visible to the naked eye, gives it an unmistakable identity, marking its presence among the platinum group metals. Furthermore, osmium holds the title of being the densest stable element known, with a density surpassing any other under normal conditions.

In terms of resistance, osmium is unmatched in several aspects. It is the purest element most resistant to compression, only surpassed by carbon in its ultra-pure diamond form. Its Mohs hardness, classified at 7, makes it extremely resistant to corrosion, heat, and mechanical deformation, even under high pressure or tension. However, this robustness is accompanied by characteristic fragility: unlike malleable metals like copper or nickel, which deform gradually under pressure, osmium, with its crystalline structure, remains rigid until it suddenly fractures from a sufficiently strong impact, without showing prior deformation.

With extraordinarily high melting and boiling points, osmium defies extreme conditions, but its brittle nature and extreme density make it mechanically difficult to work with. These characteristics relegate it to very specific applications, primarily in scientific fields, where its exoticism and rarity are valued. Considered a noble metal, and for many also a precious one, osmium is extremely scarce in the Earth's crust, which drives its price to prohibitive levels. Its combination of unique properties makes it a fascinating, albeit limited-use, material, reserved for niches where its singularity is indispensable.

Osmium stands out as a metal with exceptional corrosion resistance, even under elevated temperatures. Its chemically noble nature makes it virtually immune to acids, whether reducing or oxidizing, including aggressive mixtures such as aqua regia or concentrated hydrofluoric acid. Alkaline bases, such as bleach, also fail to affect it, and its stability extends to less hostile environments, like household detergents or aqueous media, where its resistance is absolute. This ability to remain unaltered across a wide range of chemical conditions reinforces its status as one of the most robust metals in the periodic table.

The chemical nobility of osmium explains its presence in native form in the Earth's crust, where it is found uncombined with other elements, a testament to its inertia against most corrosive agents. However, this metal has a single weak point: oxygen at high temperatures. Under these conditions, osmium reacts to form osmium tetroxide (OsO₄), a volatile compound known for its strong, unpleasant odor, as well as its toxicity. This peculiar vulnerability to oxygen in extreme states does not overshadow osmium's extraordinary durability, which makes it a material of interest for scientific and technological applications where corrosion resistance is fundamental.

Osmium, though more affordable than iridium, shares with this noble metal a unique combination of properties: exceptional hardness, outstanding corrosion resistance, and dimensional stability, especially under high temperature or mechanical deformation conditions. These characteristics make it an ideal material for specialized applications where durability is crucial. In the past, osmium was used in the manufacturing of fountain pen nibs, leveraging its wear resistance. It also excels in electrical contacts exposed to extreme temperatures and corrosive environments, such as those used in high-demand equipment. One of the most notable historical applications was its use in osmium and tungsten alloys, which were among the first to form the filaments of incandescent light bulbs, thanks to their ability to withstand intense heat without degrading.

Despite these applications, osmium's uses are limited due to its high cost and scarcity in the Earth's crust. In many cases, more accessible metals, such as tungsten, can offer similar results at a significantly lower price, which relegates osmium to technical niches where its unique properties are indispensable. Its rarity and value position it as a material of interest primarily in high-precision scientific and industrial contexts.