Osmium (Os), a chemical element, is one of the platinum metals of Groups 8–10 (VIIIb), Periods 5 and 6, of the periodic table. It is the densest naturally occurring element. Osmium is a gray-white metal that is very hard, brittle, and difficult to work with, even at high temperatures. It has the highest melting point of the platinum metals, making fusing and casting difficult. English chemist Smithson Tennant discovered osmium along with iridium in the residues of platinum ores not soluble in aqua regia. He announced its isolation in 1804 and named it for the unpleasant odour of some of its compounds (Greek osme, odour).

What does osmium look like?

Osmium is a chemical element and one of the platinum metals. It is the densest naturally occurring element. Osmium is a gray-white metal that is very hard and brittle, making it difficult to work with, even at high temperatures. According to Merriam-Webster, osmium is a blue-gray or blue-black color. Powdered osmium metal exudes a characteristic odor and, when oxidized, becomes a brown or black solid.

Where can osmium be found in nature?

Pure osmium (Os) does not occur naturally. With a crustal abundance of about 0.001 part per million, it is one of the rarest elements on Earth. Osmium can be found in native alloys with other platinum metals, such as: * Siserskite (up to 80%) * Iridosmine * Aurosmiridium (25%) * Native platinum (in slight amounts) Iridosmine and similar alloys, both natural and synthetic, are used for tips of pen nibs, surgical needles, and sparking points in engines because of their hardness and resistance to corrosion.

What are some main properties of osmium?

Osmium (Os) is a chemical element and the densest naturally occurring element. It is a gray-white metal that is very hard, brittle, and difficult to manipulate, even at high temperatures. Osmium has the highest melting point of the platinum metals, making fusing and casting difficult. Key properties of osmium: * Atomic Number 76 * Atomic Weight 190.23 * Melting Point 3,000 °C (5,432 °F) * Boiling Point Approximately 5,000 °C (9,032 °F) * Specific Gravity 22.59 (at 20 °C) * Oxidation States +2, +3, +4, +6, +8 Osmium does not occur in its pure form in nature. It is found in native alloys with other platinum metals. The English chemist Smithson Tennant discovered osmium along with iridium in 1804. Powdered osmium metal exudes a characteristic odor of the poisonous volatile tetroxide, OsO_4. Osmium tetroxide is a crystalline compound used as a catalyst, oxidizing agent, and biological fixative and stain.

What are some common uses of osmium?

Osmium is a hard, brittle, gray-white metal with the highest melting point of the platinum metals. Though pure osmium does not occur naturally, it can be found in native alloys with other platinum metals. Common uses of osmium: * Osmium alloys are used to harden other platinum metals. * Osmium and iridium alloys were formerly used for fountain pen tips and phonograph needles. * Osmium tetroxide is a crystalline compound used as a catalyst, oxidizing agent, biological fixative, and stain. * Osmium wires were used in early incandescent lamps.

Why is osmium considered one of the densest elements, and why is this important?

Osmium is considered one of the densest naturally occurring elements due to its high specific gravity . Osmium has a specific gravity of 22.59 (20 °C). Osmium is used chiefly as a hardener in alloys of platinum metals. An alloy of osmium and iridium is used for tips of fountain pens and phonograph needles, and osmium tetroxide is used in certain organic syntheses.

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A labeled diagram of the periodic table entry for osmium shows its atomic number 76, symbol Os, atomic weight 190.23, electron configuration, and physical state as a solid transition metal with a hexagonal crystal structure and weakly acidic oxides.

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Figure 7: The mass spectrum of osmium. This recorder trace was obtained with an electron multiplier detecting OsO3−. The leftmost and rightmost peaks were recorded with the detector gain set at a value 100 times that used for the rest of the spectrum; this change is marked by the change in the baseline position. The small satellite peaks to the left are those of the low abundance oxygen isotopes 17O and 18O; the osmium isotopes are, from left to right, 184Os, 186Os, 188Os, 189Os, 190Os, and the satellite peaks of 192Os.

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Related Topics:: chemical element; transition metal; ruthenium; osmium-187; osmium-186

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osmium

chemical element

Also known as: Os

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Last updated

Sep. 29, 2026 •History

Top Questions

What is osmium?
Osmium (Os), a chemical element, is one of the platinum metals of Groups 8–10 (VIIIb), Periods 5 and 6, of the periodic table. It is the densest naturally occurring element.

Osmium is a gray-white metal that is very hard, brittle, and difficult to work with, even at high temperatures. It has the highest melting point of the platinum metals, making fusing and casting difficult.

English chemist Smithson Tennant discovered osmium along with iridium in the residues of platinum ores not soluble in aqua regia. He announced its isolation in 1804 and named it for the unpleasant odour of some of its compounds (Greek osme, odour).
3 Britannica Sources
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What does osmium look like?
Osmium is a chemical element and one of the platinum metals. It is the densest naturally occurring element.

Osmium is a gray-white metal that is very hard and brittle, making it difficult to work with, even at high temperatures. According to Merriam-Webster, osmium is a blue-gray or blue-black color. Powdered osmium metal exudes a characteristic odor and, when oxidized, becomes a brown or black solid.
2 Britannica Sources
1. osmium
2. Definition of osmium dioxide
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Where can osmium be found in nature?
Pure osmium (Os) does not occur naturally. With a crustal abundance of about 0.001 part per million, it is one of the rarest elements on Earth.

Osmium can be found in native alloys with other platinum metals, such as:
* Siserskite (up to 80%)
* Iridosmine
* Aurosmiridium (25%)
* Native platinum (in slight amounts)

Iridosmine and similar alloys, both natural and synthetic, are used for tips of pen nibs, surgical needles, and sparking points in engines because of their hardness and resistance to corrosion.
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What are some main properties of osmium?
Osmium (Os) is a chemical element and the densest naturally occurring element. It is a gray-white metal that is very hard, brittle, and difficult to manipulate, even at high temperatures. Osmium has the highest melting point of the platinum metals, making fusing and casting difficult.

Key properties of osmium:
* Atomic Number 76
* Atomic Weight 190.23
* Melting Point 3,000 °C (5,432 °F)
* Boiling Point Approximately 5,000 °C (9,032 °F)
* Specific Gravity 22.59 (at 20 °C)
* Oxidation States +2, +3, +4, +6, +8

Osmium does not occur in its pure form in nature. It is found in native alloys with other platinum metals. The English chemist Smithson Tennant discovered osmium along with iridium in 1804. Powdered osmium metal exudes a characteristic odor of the poisonous volatile tetroxide, OsO_4. Osmium tetroxide is a crystalline compound used as a catalyst, oxidizing agent, and biological fixative and stain.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are some common uses of osmium?
Osmium is a hard, brittle, gray-white metal with the highest melting point of the platinum metals. Though pure osmium does not occur naturally, it can be found in native alloys with other platinum metals.

Common uses of osmium:
* Osmium alloys are used to harden other platinum metals.
* Osmium and iridium alloys were formerly used for fountain pen tips and phonograph needles.
* Osmium tetroxide is a crystalline compound used as a catalyst, oxidizing agent, biological fixative, and stain.
* Osmium wires were used in early incandescent lamps.
2 Britannica Sources
1. osmium
2. Definition of osmium tetroxide
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Why is osmium considered one of the densest elements, and why is this important?
Osmium is considered one of the densest naturally occurring elements due to its high specific gravity. Osmium has a specific gravity of 22.59 (20 °C).

Osmium is used chiefly as a hardener in alloys of platinum metals. An alloy of osmium and iridium is used for tips of fountain pens and phonograph needles, and osmium tetroxide is used in certain organic syntheses.
2 Britannica Sources
1. osmium
2. Definition of osmium
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osmium (Os), chemical element, one of the platinum metals of Groups 8–10 (VIIIb), Periods 5 and 6, of the periodic table and the densest naturally occurring element. A gray-white metal, osmium is very hard, brittle, and difficult to work, even at high temperatures. Of the platinum metals, it has the highest melting point, so fusing and casting are difficult. Osmium wires were used for filaments of early incandescent lamps before the introduction of tungsten. It has been used chiefly as a hardener in alloys of the platinum metals, though ruthenium has generally replaced it. A hard alloy of osmium and iridium was used for tips of fountain pens and phonograph needles, and osmium tetroxide is used in certain organic syntheses.

Pure osmium metal does not occur in nature. Osmium has a low crustal abundance of about 0.001 part per million. Though rare, osmium is found in native alloys with other platinum metals: in siserskite (up to 80 percent), in iridosmine, in aurosmiridium (25 percent), and in slight amounts in native platinum. Processes for isolating it are an integral part of the metallurgical art that applies to all platinum metals.

The English chemist Smithson Tennant discovered the element together with iridium in the residues of platinum ores not soluble in aqua regia. He announced its isolation (1804) and named it for the unpleasant odour of some of its compounds (Greek osme, odour).

Concept artwork on the periodic table of elements.

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Of the platinum metals, osmium is the most rapidly attacked by air. The powdered metal, even at room temperature, exudes the characteristic odour of the poisonous volatile tetroxide, OsO₄. Because solutions of OsO₄ are reduced to the black dioxide, OsO₂, by some biological materials, it is sometimes used to stain tissues for microscopic examinations.

Osmium is, with ruthenium, the most noble of the platinum metals, and cold and hot acids are without effect on them. It can be dissolved by fused alkalies, especially if an oxidizing agent such as sodium chlorate is present. Osmium will react at 200 °C with air or oxygen to form OsO₄.

Osmium exhibits oxidation states from 0 to +8 in its compounds, with the exception of +1; well-characterized and stable compounds contain the element in +2, +3, +4, +6, and +8 states. There are also carbonyl and organometallic compounds in the low oxidation states −2, 0, and +1. Ruthenium is the only other element known to have an oxidation state of 8. (The chemistries of ruthenium and osmium are generally similar.) All compounds of osmium are easily reduced or decomposed by heating to form the free element as a powder or sponge. There is an extensive chemistry of the tetroxides, oxohalides, and oxo anions. There is little, if any, evidence that simple aquo ions exist, and virtually all their aqueous solutions, whatever the anions present, may be considered to contain complexes.

Natural osmium consists of a mixture of seven stable isotopes: osmium-184 (0.02 percent), osmium-186 (1.59 percent), osmium-187 (1.96 percent), osmium-188 (13.24 percent), osmium-189 (16.15 percent), osmium-190 (26.26 percent), and osmium-192 (40.78 percent).

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Element Properties
atomic number	76
atomic weight	190.23
melting point	3,000 °C (5,432 °F)
boiling point	about 5,000 °C (9,032 °F)
specific gravity	22.59 (20 °C)
oxidation states	+2, +3, +4, +6, +8
electron configuration	[Xe]4f ¹⁴5d⁶6s²

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Rick Livingston.

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