Osmium
| Osmium | |
|---|---|
| |
| Atomic number | 76 |
| Symbol | Os |
| Boiling point | 5012 °C |
| Electron configuration | [Xe] 4f14 5d6 6s2 |
| Density | 22.59 g/cm^3 |
| Main isotopes | 189Os, 190Os, 192Os |
| Melting point | 3033 °C |
| Block | d |
| Phase STP | Solid |
| Oxidation states | +4, +6, +8 |
| Wikidata | Q751 |
Osmium (Os, atomic number 76) is a hard, brittle, bluish-white transition metal of the platinum group It is a dense, lustrous solid at room temperature and pressure. Osmium is extraordinarily rare in Earth’s crust (on the order of 50 parts per trillion by mass and has the highest measured density of any stable element (about 22.6 g/cm³) In the periodic table, osmium lies in group 8, period 6 (bottom of its column) and is part of the d-block. Its elemental symbol is Os. Common oxidation states of osmium range from about –2 up to +8, with +2, +3, +4 and +8 being most frequently encountered In chemical compounds, osmium often exhibits +2, +3, and +4 states; the +8 state appears only in the volatile oxide OsO₄. At standard conditions (STP), osmium is a solid.
Atomic Structure and Periodic Trends
A neutral osmium atom has 76 protons and 76 electrons, with electronic configuration [Xe] 4f¹⁴ 5d⁶ 6s² (The filled 4f shell is inner, while the 5d⁶6s² electrons lie in its valence shell.) In simple terms, osmium contributes six valence electrons (the 5d and 6s electrons) in bonding. As a heavy late-transition metal, osmium’s atomic radius is relatively small. The empirical atomic radius is about 130 picometers Reflecting its high nuclear charge and effective electron shielding, osmium has a moderately high first ionization energy (about 8.7 eV for a metal, and a Pauling electronegativity of ~2.2 These values are comparable to those of its platinum-group neighbors (ruthenium, iridium), consistent with periodic trends: down the group the ionization energy generally increases and atomic radius slightly decreases.
Overall, osmium’s atomic properties reflect its position at the bottom of group 8: it has a heavy nucleus and tightly held d-electrons. Its compact radius and high ionization energy make it chemically less reactive (noble-like) than lighter congeners, yet able to attain unusually high oxidation states (unlike iron). Valence electrons fill the 5d and 6s orbitals, and relativistic effects also contribute to its metallic character.
Isotopes and Nuclear Properties
Osmium has seven naturally occurring isotopes, with mass numbers 184, 186, 187, 188, 189, 190, and 192 Of these, five decay so slowly as to be effectively stable: ^184Os, ^187Os, ^188Os, ^189Os, and ^192Os (with ^192Os being the most abundant) The remaining two (^186Os and ^190Os) are primordial radioisotopes that undergo alpha decay, but with extremely long half-lives (on the order of 10^15 to 10^19 years) for practical purposes all seven are treated as stable. Over 30 artificial (radioactive) isotopes of osmium are known, with mass numbers from about 160 to 203. The longest-lived synthetic isotope is ^191Os, with a half-life around 6.0 years (others have much shorter half-lives on the order of hours or days).
Two osmium isotopes of note have potential applications in geoscience: ^187Os (nuclear spin ½) and ^189Os (spin 3/2) In particular, ^187Os is the daughter product of ^187Re (rhenium) decay (half-life ~4.1×10¹⁰ years) and is used in rhenium–osmium radiometric dating of rocks and meteorites The high Z of osmium also means its nuclear moments are small, so osmium’s NMR activity is very weak (making ^187Os NMR studies difficult). In natural abundance, ^187Os stands at ~1.6%, and ^189Os ~16.1%. No significant naturally occurring osmium radioactivity (beyond the minute alpha decay mentioned) affects its practical use.
Allotropes and Typical Compounds
Osmium does not have multiple elemental allotropes in the way carbon or phosphorus do. The metal crystallizes in a single form at ambient conditions, namely a hexagonal close-packed (hcp) lattice (No other metallic phase or nonmetallic form of osmium is known under ordinary conditions.) The hcp structure gives osmium its high density.
In chemistry, osmium forms a wide variety of compounds across its possible oxidation states. A very distinctive compound is osmium tetroxide, OsO₄, in which osmium is in the +8 oxidation state. This four-coordinate molecular compound is volatile, colorless (or pale yellow), and is one of the strongest oxidizing agents known OsO₄ forms readily when osmium metal is exposed to air; its pungent smell (reminiscent of chlorine or garlic) inspired the element’s name (from Greek osme “odor”) Due to its volatility and toxicity, OsO₄ is rarely encountered outside specialized laboratory or industrial use.
Another important oxide is osmium dioxide (OsO₂), which is a stable black crystalline solid with osmium in the +4 state Lower oxides (e.g. OsO and Os₂O₃) are known in laboratory conditions but are less common. Osmium also forms “mate” anions such as OsO₄²⁻ in basic conditions.
Halogenides of osmium are known across several oxidation states. For example, osmium hexafluoride (OsF₆) is a yellow, volatile solid (Os in +6), and osmium forms higher fluorides (OsF₇) and even [OsF₈]^2– species Chlorides include OsCl₄ and OsCl₅ (both solid), among others Osmium bromides (e.g. OsBr₄) and iodides (OsI, OsI₂, OsI₃) are also characterized In these halides, osmium exhibits +2, +3, +4, +6, +7 oxidation states depending on the ligand. No simple stable hydride (Os–H) is known – osmium metal does not react with hydrogen to form a classical hydride
Osmium also enters coordination and organometallic chemistry. Classic carbonyl clusters are known (for instance Os₃(CO)₁₂, an analog of Fe3(CO)₁₂) The bis(cyclopentadienyl) complex “osmocene”, Os(C₅H₅)₂, is analogous to ferrocene. Complexes with amine or arene ligands (e.g. OsCl₂(NH₃)₅) exist as well. Osmium’s rich organometallic chemistry supports catalysts and research compounds.
In summary, osmium’s typical compounds include strong oxides (especially osmium tetroxide), various halides and teased oxidizing complexes, and carbonyl/organometallic clusters. These demonstrate its ability to adopt high oxidation states with strongly electronegative ligands, while lower oxidation states (0, +2) are favored in carbonyls or with strong donor ligands.
Physical Properties
Osmium metal is extremely dense and refractory. Its density at room temperature is about 22.59 g/cm³ – about four times that of iron and denser than the more familiar platinum (21.45 g/cm³). In fact, among elements with stable isotopes, osmium’s density is the highest known This arises from its tightly packed atoms in the hcp lattice. Osmium is a hard and very incompressible metal; its melting point is extremely high (3306 K, or 3033 °C) and its boiling point is around 5281 K (5008 °C) These values place osmium among the highest melting elements in the periodic table (fourth highest, after carbon, tungsten, and rhenium
At room temperature osmium appears as a bluish-gray, glossy metal It is quite hard (brittle and difficult to machine); in fact osmium metal shavings or powders are known to ignite or form oxides on exposure to air (requiring caution). Like most metals, osmium is a good conductor of electricity and heat, although its thermal conductivity (~88 W/m·K) is modest compared to lighter metals. Electronically, it is a paramagnetic metal at ambient temperature (no ferromagnetism).
Spectroscopically, neutral osmium atoms have rich spectral lines (in the visible and UV), but one often notes the lack of colorful compounds outside of a few complexes. Instead, its physical appearance is quite metallic and subdued.
Overall, osmium metal is characterized by very high density, high melting/boiling points, hcp crystal structure and mechanical hardness. These extreme properties limit its uses to where such durability or density is specifically required.
Chemical Reactivity and Trends
Chemically, elemental osmium is unusually inert among metals, reflecting its noble nature in the platinum group. In bulk, metallic osmium resists corrosion by most chemicals. It is unaffected by dilute acids and even resists attack by the potent oxidizing mixture aqua regia Scarcely anything, short of harsh conditions, will dissolve osmium metal. For example, cold alkalis and weak acids do nothing to osmium. Only strong oxidizers or high-energy processes oxidize it. For instance, fusion with alkali and peroxide or chlorate yields soluble osmate salts (e.g. potassium osmate), and osmium metal can be dissolved by molten hydroxides with an oxidizer Very hot concentrated nitric acid oxidizes osmium to volatile OsO₄ and molten chlorine can burn it to form chlorides at high temperature.
When it does react, osmium follows trends of late group 8 metals but to higher extremes. It can attain the maximum oxidation state of +8 (in OsO₄) because of its large number of d-electrons and relativistic stabilization, whereas iron (same group) only goes to +6 (in ferrates). Osmium also forms stable lower oxidation states (+2, +3, +4) in many complexes. It shows strong affinity for π-acid ligands (like CO, NO, polypyridines) which stabilize low-valent Os, and strong oxidizing ligands (O, F, chlorine) which stabilize high-valent Os.
In redox terms, osmium and its compounds are strong oxidants. Osmium tetroxide (OsO₄) is itself a powerful oxidizing agent: it readily adds across carbon–carbon double bonds, cleaves olefins, and oxidizes organic functional groups. In fact, OsO₄ is famously used as a reagent in organic chemistry (its ability to dihydroxylate alkenes underlies the Sharpless asymmetric dihydroxylation reaction Soluble osmate anion OsO₄²⁻ can also oxidize many substrates. In comparison, osmium metal has a negative standard electrode potential (roughly –0.7 V vs SHE in the OsO₄/Os couple), reflecting that the Os(0) state is stable and its oxidation to higher states requires strong oxidants.
Contrast this with, say, iron or copper: osmium never shows the equivalent +1 or +2 ease of transition like iron(II) or copper(II) in water. Osmium instead is “higher in the reactivity series” in the sense of being nobler (less reactive) toward common reagents. It resists chlorine gas at room temperature, but hot chlorine will form OsCl₄, OsCl₅, etc. Aqua regia (which dissolves gold and platinum) leaves osmium largely intact, precipitating a dark residue (that residue, as Tennant discovered, contained osmium and iridium)
In minerals processing, this chemical inertness is exploited: osmium and ruthenium remain undissolved while platinum-group elements dissolve, allowing separation by distilling osmium tetroxide or selective extraction
In acids, osmium does not behave as a classical strong acid or base. Instead, it is amphoteric: it can form complex osmium anions with strong base (osmates) and it forms polyoxo-species in very oxidizing conditions. It never forms stable aqueous cations like many metals do.
Overall, osmium’s chemistry is defined by its resistance to everyday reagents and the dominance of its highest oxide as an oxidizing agent. It stands at the rugged end of the transition metals – more inert than iron or nickel yet capable of unusually high oxidation states under strong oxidation.
Occurrence and Production
In nature, osmium is extremely scarce. Its average abundance in Earth’s crust is only about 50 parts per trillion by mass Like other platinum-group metals, osmium occurs in ultra-mafic igneous deposits and in alluvial platinum ores. It is not found free in nature (except in trace amounts of unalloyed grains); rather, it is usually alloyed with other PGMs or hosted in mineral lattices. Major geological sources of osmium (as with the other PGMs) include layered intrusions such as the Bushveld Complex in South Africa, the Norilsk deposits in Russia, and to a lesser extent some platinum-bearing sands in Colombia and gold-copper deposits in Canada These deposits yield platinum, palladium, iridium, rhodium and sometimes osmium and ruthenium.
Commercially, osmium is obtained almost exclusively as a byproduct of refining nickel, copper, or platinum ores. For example, when nickel-copper sulfide ores are electrolytically refined (as in Sudbury, Canada, or Norilsk, Russia), platinum-group elements collect in the anode slime. This PGM-rich residue is then processed chemically. Typically, the metals are dissolved in chlorinated or nitric acids; osmium (and ruthenium) oxidize to OsO₄ and RuO₄, which are volatile. The osmium tetroxide is distilled off (or extracted into solvent) One can also precipitate osmium from solution as ammonium hexachloroosmate(IV) and then reduce it with hydrogen to osmium metal Either way, the key is that osmium, unique among the group, can be separated via its volatile oxide.
Production of metallic osmium is very limited. Because demand is small, only a few hundred to a few thousand kilograms are produced worldwide each year Osmium is seldom traded on commodity markets; it is often grouped with “minor platinum metals” in statistics. For example, U.S. imports of osmium averaged roughly 150 kg per year in 2014–2021 The leading sources today remain the PGM producers of South Africa and Russia.
The metal used in industry is obtained either as a powder or sponge which can be pressed or sintered into parts. Alloying takes place with other PGM metals to make extremely hard materials. Osmium’s high density and hardness mean that conventional machining is difficult; it is more often used in tiny parts (alloy tips) rather than bulk components.
Applications and Technology
Because osmium is rare and toxic in oxidized form, its applications are highly specialized. The metal’s exceptional hardness and wear resistance (even when alloyed at a few percent) is leveraged in small but important ways. Osmium alloys (often with iridium and platinum) are used to make electrical contacts, tip points, and pivots that endure frequent use Classic examples include tips of fountain pen nibs, gramophone stylus tips, and precision instrument bearings. These alloys (sometimes called osmiridium) exploit osmium’s hardness to resist abrasion. Small amounts of osmium in platinum-group alloy improve the durability without greatly affecting corrosion resistance.
One of osmium’s dominant technological roles is through its volatile oxide, osmium tetroxide (OsO₄). Despite being extremely toxic, OsO₄ is widely used in scientific and medical fields. It is a powerful biological stain: osmium tetroxide reacts with unsaturated lipids and proteins, darkening them. In electron microscopy, OsO₄ is used to fix and stain biological tissues, providing contrast in specimens (due to osmium’s high atomic number) that would otherwise be mainly carbon and hydrogen It is also used as a stain for light microscopy (e.g. staining fatty tissues) and even in forensic science to develop fingerprints.
In chemical manufacturing, osmium tetroxide is a well-known reagent. It catalyzes the syn-dihydroxylation of alkenes (the Sharpless asymmetric dihydroxylation) Because OsO₄ selectively adds oxygen across double bonds, it’s valuable in organic synthesis despite its cost. (Karl Barry Sharpless won a Nobel Prize in 2001 for exploiting this reaction in asymmetric synthesis.) Osmium catalysts have also been used for other oxidations; however, many of these uses give way to cheaper alternatives (e.g. manganese or ruthenium catalysts) when large scale or cost sensitivity arises.
Other niche uses include very few osmium-based organometallics being investigated as anticancer drugs The rationale is that osmium complexes can bind DNA or proteins and kill cancer cells (similar to platinum-based drugs). This is early-stage research and osmium drugs are not yet clinically used.
Historically, osmium had some early applications: its metal filaments were briefly tried in incandescent lamps (around 1900) due to high melting point, but tungsten filaments soon replaced it. It was also used in some 19th- and 20th-century fountain pen alloys (leading to the name “Osram” for an early lamp company, combining Osmium and Wolfram, the German for tungsten Today, however, the cost and toxicity of osmium limit it to the small-scale uses above. Essentially all osmium demand comes from specialized industries (electronics, microscopy, niche chemistry), and minute quantities of OsO₄ reagent.
Biology, Environment, and Safety
Osmium has no known biological role in living organisms. In nature, its extreme rarity and inertness mean virtually no osmium biology exists. The prime safety concern with osmium is chemical toxicity, primarily via osmium tetroxide. Pure osmium metal at room temperature is relatively inert and not immediately hazardous; however, its surface slowly oxidizes in air. Even trace formation of osmium tetroxide (a volatile gas) poses serious danger.
Osmium tetroxide is highly toxic and corrosive. It reacts with organic tissue on contact, causing severe burns to the skin, eyes, and respiratory tract. Its vapor has a sharp, penetrating odor (hence the name) and can cause blindness or lung damage. Safety guidelines strictly limit exposure: for example, OSHA/NIOSH permit only about 0.002 mg/m³ (0.0002 ppm) as an 8-hour time-weighted average A single drop of concentrated OsO₄ on the skin can cause injury. Because of this, osmium handling requires glove boxes and protective gear. Any laboratory work with osmium ores or powder takes precautions to prevent oxide formation in the air.
Environmental impact of osmium is very limited due to its scarcity. Any osmium entering ecosystems would likely deposit as inert metal or oxide particles, with unknown long-term effects. Osmium tetroxide in the environment breaks down (water/air) to less harmful compounds over time. There are no widespread osmium compounds in soils or water under normal conditions. Disposal of osmium-containing waste requires care to neutralize OsO₄ (e.g. with reducing agents).
In summary, osmium’s only real hazard is its chemistry: OsO₄ is a potent oxidizer and poison Otherwise, the element poses minimal biological exposure risk (it is insoluble and immobile as metal). Standard safety data sheets classify osmium tetroxide as “very toxic,” with risk phrases such as “contact with eyes, skin, or inhalation is dangerous,” reflecting the extreme toxicity of osmium’s compounds.
History and Etymology
Osmium was discovered in 1803 by the English chemist Smithson Tennant. Working with residues from platinum ore, Tennant found that a portion of the black insoluble material was a new element He oxidized it to a yellow crystalline oxide and noted its strong smell. He named the element “osmium” from the Greek osme meaning “smell” or “odor,” referring to the pungent sniff of osmium tetroxide The element’s name first appeared in a letter to the Royal Society on June 21, 1804 Tennant’s work showed that osmium was distinct from iridium and platinum, which had also been isolated from the same platinum ore.
Early on, osmium’s most famous compound became OsO₄, due to its striking properties. In fact, OSHA regulations and toxicology studies have been concerned largely with osmium tetroxide. Osmium metal itself remained a laboratory curiosity for much of the 19th century. Refractory platinum-is-group materials like osmium and iridium were sometimes mentioned in early industrial chemistry histories (for example, they were tried as catalysts in the Haber ammonia process) but cheaper materials soon prevailed.
The 20th century saw the first practical uses of osmium alloys in engineering (pen nibs, phonograph needles, etc.). The famed electric lightbulb era briefly tested osmium filaments, but tungsten won out due to cost and stability (tungsten’s higher abundance and higher melting point made it superior). In cultural terms, osmium entered the language only via technical trade names (e.g. osmiridium for osmium-iridium alloys, or osmonium tetroxide as a lab reagent). The early 1900s German lamp company Osram (co-founded in 1906) chose its name by combining Os for osmium and Wolfram (tungsten) reflecting the two elements.
In summary, osmium’s history is tied to analytical chemistry and its remarkable oxide. Discovered by Tennant in 1803 as a “smelly” oxide, it has remained a curiosity with highly specialized applications. Its naming is one of few elements (like phosphorus, hydrogen) derived from Greek descriptive terms (“odor mask”). Osmium was among the earliest elements isolated in modern chemistry and continues to be referenced in high-precision science (catalysis, microscopy) rather than broad industry.
Data Table
| Property | Value |
|---|---|
| Symbol | Os |
| Atomic number (Z) | 76 |
| Element category | Transition metal (platinum group), d-block |
| Group / Period / Block | 8 / 6 / d |
| Atomic weight (amu) | 190.23 |
| Electron configuration | [Xe] 4f¹⁴ 5d⁶ 6s² |
| Valence electrons | 6 (the 5d and 6s electrons) |
| Common oxidation states | –2, –1, +1, +2, +3, +4, +6, +7, +8 (most common +2, +3, +4, +8) |
| Phase at STP | Solid metal |
| Density (20 °C) | 22.59 g/cm³ |
| Atomic radius (empirical) | ~130 pm |
| Paulig electronegativity | 2.20 (Pauling scale) |
| First ionization energy | 8.7 eV (836 kJ/mol) |
| Melting point | 3306 K (3033 °C) |
| Boiling point | 5281 K (5008 °C) |
| Crystal structure | Hexagonal close-packed (hcp) |
| Mohs hardness | ~7 (very hard, brittle) |
| Stable isotopes | ⁹²Os, ¹⁸⁹Os, ¹⁸⁸Os, ¹⁸⁷Os, ¹⁸⁴Os |
| Natural abundance (crust) | ~50 ppt (parts per trillion) |