Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Osmium is an element in Group 8 (VIIIB) of the periodic table. The periodic table is a chart showing how chemical elements are related to one another. Osmium is also a member of the platinum family. This family consists of five other elements: ruthenium, rhodium, palladium, indium, and platinum. These elements often occur together in the Earth's crust. They also have similar physical and chemical properties, and are used in alloys.
Osmium was discovered in 1804 by English chemist Smithson Tennant (1761-1815). Tennant found the new element in an ore of platinum.
Osmium is a very rare element and has few commercial uses. Osmium tetroxide (OsO4), is more widely used, however, because it is so active.
Group 8 (VIIIB)
Discovery and naming
Platinum metal (atomic number 78) was known to chemists as early as 1741. Over the next 60 years, however, scientists discovered that the substance they knew as "platinum" was usually a mixture of substances. These substances proved to be new elements. Osmium was one of the new elements discovered in impure platinum.
In the early 1800s, Smithson Tennant was studying platinum. He found that a black powder remained when platinum was dissolved in aqua regia. Aqua regia is a mixture of hydrochloric and nitric acids. The term aqua regia means "royal water." It often dissolves materials that either add by itself does not dissolve.
In 1804, Tennant announced that the black powder was actually a mixture of two new elements. He called them indium and osmium. He suggested osmium's name because of the unusual smell of the compound he was working with, osmium tetroxide. Osmium comes from the Greek word osme, meaning "odor."
Osmium is a bluish-white, shiny metal with a melting point of about 3,000°C (5,400°F) and a boiling point of about 5,500°C (9,900°F). Its density is 22.5 grams per cubic centimeter. These numbers are the highest of any platinum metal. They are also among the highest of all elements.
Osmium is unworkable as a metal. It cannot be melted and shaped like most metals. Because it is unworkable, it has very few practical uses.
Osmium is dissolved by acids or by aqua regia only after long periods of exposure to the liquids. When heated, the metal combines with oxygen to form osmium tetroxide (OsO4). Osmium tetroxide is very toxic and the only important commercial compound of osmium.
Occurrence in nature
Osmium is very rare. Its abundance is thought to be about 0.001 parts per million (one part per billion). That places the element among the half dozen least abundant elements in the Earth's crust.
The most common ore of osmium is osmiridium. The element also occurs in all ores of platinum.
There are seven naturally occurring isotopes of osmium. The most abundant are osmium-192, osmium-190, and osmium-189 These three isotopes make up 41.0 percent, 26.4 percent, and 16.1 percent of natural osmium, respectively. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
Many radioactive isotopes of osmium are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive. No radioactive isotope of osmium has any important use.
Osmium is obtained when platinum metal is extracted from its ores.
Osmium metal has few uses. It is sometimes added to platinum or indium to make them harder. Some of the best pen points, for example, are made of osmium-platinum alloys. An alloy is made by melting or mixing two or more metals. The osmium-platinum alloy is harder than pure platinum. Some alloys of osmium and platinum are also used to make specialized laboratory equipment.
Finely divided osmium metal is also used as a catalyst. A catalyst is a substance used to speed up or slow down a chemical reaction. The catalyst does not undergo any change itself during the reaction. The process for making ammonia from combined hydrogen and nitrogen sometimes uses osmium as a catalyst.
Osmium tetroxide (OsO4) is in demand for use as a catalyst for research purposes. The problem is that this compound of osmium is very dangerous to use. It is shipped in small glass containers called ampules. The ampules carry no labels, nor are they marked with ink. The label and ink would react violently with osmium tetroxide. Users are instructed to open and use an ampule containing osmium tetroxide with great care.
Osmium tetroxide is so dangerous to use that it is shipped in a small glass container. The container carries no label or ink because each would react violently with the compound.
Some compounds of osmium are extremely dangerous. They irritate the respiratory passage (throat, lungs, etc.), the skin, and the eyes. They must be handled with extreme care. This caution is especially important for the most widely used compound of osmium, osmium tetroxide.
The element osmium was discovered in 1804 by English chemist Smithson Tennant (1761–1815) in the black residue that remained after crude platinum was dissolved in aqua regia. The average abundance in Earth's crust is very low, about 0.005 grams (0.00018 ounces) per metric ton, and only four osmium-containing minerals, all extremely rare, are known: erlichmanite, OsS2; omeiite, (Os,Ru)As2; and osarsite and anduoite, (Os,Ru)AsS. Osmium also occurs in natural alloys with iridium and/or ruthenium (e.g., iridosmium). Osmium is obtained as a by-product of refining nickel and the more common platinum group metals . Worldwide production is very small, approximately 500 kilograms (1,102 pounds) per year (versus 2,500,000 kilograms, or 5,512,000 pounds, per year for gold). Despite its rarity, osmium is only 30 percent more expensive than gold because it has few commercial uses. Osmium metal is lustrous, bluish-white, hard, and brittle; it melts at 3,127°C (5,661°F) and boils at 5,303°C (9,577°F). It is the densest element known: Its density is 22.59 grams (0.8 ounces) per cubic centimeter (twice that of lead). Osmium is combined with other platinum group elements to yield extremely hard alloys, which find limited use as electrical contacts, wear-resistant instrument pivots and bearings, and tips for high-priced ink pens. Osmium forms compounds in all of its oxidation states, from +8 to −2. Its chemistry closely resembles that of ruthenium. The most important compound is osmium tetroxide, OsO4, a pale yellow solid used as a stain in microscopy, in fingerprint detection, and as a catalyst in the production of some pharmaceuticals. Osmium tetroxide has an unpleasant chlorinelike odor, which prompted Tennant to name the element using the Greek word osme, "a smell."
Gregory S. Girolami
Winter, Mark. "WebElements." Available from <http://www.webelements.com>.
os·mi·um / ˈäzmēəm/ • n. the chemical element of atomic number 76, a hard, dense, silvery-white metal of the transition series. (Symbol: Os)