Osmium (Os)

• Element Symbol: Os
• Atomic Number: 76
• Atomic Mass: 190.23
• Group # in Periodic Table: 8
• Group Name: Transition metals
• Period in Periodic Table: 6
• Block of Periodic Table: d-block
• Discovered by: Smithson Tennant (1803)

Osmium is a transition metal whose symbol is Os and whose atomic number is 76. This metallic element is hard, brittle, and bluish-white in color. Osmium is the densest naturally occurring element. It belongs to the platinum group. This group of six metallic elements can be found in the very center of the periodic table. The other five platinum group metals are ruthenium, rhodium, palladium, iridium, and platinum. When combined into alloys, these six transition metals have many practical uses.

Osmium was discovered by English chemist Smithson Tennant in 1803. Its discovery coincided with that of another platinum group metal, iridium. These two elements were first produced when Tennant took a sample of crude platinum and dissolved it in a mixture of nitric acid and hydrochloric acid (also known as aqua regia). The result was a metallic black powder. Tennant then took this residue and subjected it to a series of purifications. First, he treated the powder with sodium hydroxide (lye) and heat. Then he added water to the remaining solution to remove any alkaline (basic) compounds. Finally, to remove any acidic compounds, he added hydrochloric acid to what was left over from the second purification. At the very end of the procedure, Tennant found osmium in the basic solution and iridium in the acidic solution. He named the first element "osmium" after the Greek word osme (smell) because it had a very unpleasant odor.

Physical Properties

Osmium is a very hard, easily breakable (brittle) transition metal. Its Mohs hardness scale rating is 7.0. Osmium metal is bluish-white in color. It is the densest of all the elements, with a density of 22.59 grams per cubic centimeter (g/cm³) at room temperature. Osmium has the highest melting point and boiling point of the platinum group—3030 degrees Celsius (°C) and 5012 °C, respectively. Also, it has the fourth-highest melting point of all the elements. Osmium’s specific heat is 130 joules per kilogram-kelvin (J/kg·K). Its thermal conductivity is 87.6 watts per meter-kelvin (W/m·K). Since it is a metal, its standard state at 298 kelvins (K) is solid. Due to these physical properties, osmium is extremely difficult to work with and use in practical applications.

Chemical Properties

Osmium has a wide range of oxidation states: −2 to +8. This is the widest range of any element. The most common ones are +2, +3, +4, and +8. The +8 oxidation state is noteworthy because it is the highest attained by any element except for iridium, which has a +9 oxidation state. Due to this wide range of oxidation states, osmium forms compounds with several different elements, including the gases oxygen, nitrogen, hydrogen; the nonmetal carbon; the metal sodium; and the halogens iodine, bromine, chlorine, and fluorine

Osmium has thirty-four isotopes with known half-lives. They have mass numbers ranging from 162 to 196. Naturally occurring osmium is a mixture of seven different isotopes. In order of increasing abundance, they are osmium-184 (0.02 percent), osmium-186 (1.6 percent), osmium-187 (2.0 percent), osmium-188 (13.2 percent), osmium-189 (16.1 percent), osmium-190 (26.3 percent), and osmium-192 (40.8 percent). Six of these isotopes are totally stable, but osmium-186 is considered to be slightly unstable because it decays after a half-life of 2.0 × 10¹⁵ years.

Applications

Osmium is the least abundant of the stable elements found in Earth’s crust. Specifically, it makes up just 50 parts per trillion of the continental crust. It can be found in nature either as an uncombined element or in natural alloys. These natural alloys can contain iridium, nickel, copper, or the sulfides. Within Earth’s crust, osmium, like iridium, is found at its highest concentrations in three different types of geologic structures: igneous deposits, impact craters, and deposits reworked from one of the two other structures. The largest known primary reserves of osmium are found in the Bushveld igneous complex in South Africa. Other significant sources of osmium include the large copper-nickel deposits found near Norilsk in Russia and in the Sudbury Basin in Canada. Smaller reserves can be found in the United States. Yet another source for osmium is the alluvial deposits in Colombia, which are still used as a source for all of the platinum group metals. The second-largest alluvial deposit, which can be found in the Ural Mountains of Russia, is still mined today. Osmium, along with the rest of the platinum-group metals, is obtained commercially as a by-product from nickel and copper mining and processing.

Due to the fact that osmium is so difficult to work with, it has only a few practical commercial applications. It is used to produce very hard alloys that are in turn used for commercial products such as fountain pen tips, instrument pivots, needles, and electrical contacts. Osmium also has a few different scientific applications. This transition metal is used in the chemical industry as a catalyst to speed up certain slow chemical reactions. As for its biological applications, the compound osmium tetroxide (OsO₄) has a couple of uses. First, it has been used in fingerprint detection. Second, osmium tetroxide is often used as a stain for fatty tissue that is being studied under an optical or electron microscope because it is a strong oxidizing agent. The main mechanism of the compound works by cross-linking lipids when it reacts with unsaturated carbon-carbon bonds. This cross-linking both fixes biological membranes in place in tissue samples and simultaneously stains them. Since osmium atoms are extremely electron dense, osmium staining significantly enhances image contrast in transmission electron microscopy (TEM) studies of biological materials. Another osmium compound, osmium ferricyanide (OsFeCN), exhibits similar fixing and staining action.

Bibliography

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Haynes, William M., ed. CRC Handbook of Chemistry and Physics. 95th ed. Boca Raton: CRC, 2014. Print.

Kaye & Laby Tables of Physical & Chemical Constants. Natl. Physical Laboratory, 2015. Web. 25 Sept. 2015.

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