What is Osmium?
The chemical element osmium having atomic number 76 and symbol Os was discovered by Smithson Tennant in 1803. It is brittle and hard in nature, and its colour is bluish-white. This element is a transition metal present in the group of platinum and found mainly in platinum ores.
Osmium density is 22.59 g/cm3 (experimentally measured with the help of x-ray crystallography), and it is the densest element which occurs naturally. Also, it is the rarest abundant element available in the crust of the Earth.
Let us proceed with its characteristics.
Characteristics of Osmium
Like all other elements, osmium also has both chemical and physical properties. Let us first begin with the chemical ones.
Compounds are formed by osmium whose oxidation states range from -2 to +8. The most natural oxidation states are +2, +3, +4 and +8. Among these four states, +8 is noteworthy because it is the largest that is attained by a chemical element apart from iridium’s +9, and is experienced only in ruthenium, xenon, iridium and hassium. -1 and -2 oxidation states shown by two readily participating compounds Na2[Os4(CO)13] and Na2[Os(CO)4 ] are utilised in osmium cluster compound synthesis.
Osmium tetroxide is the most common chemical compound having oxidation state +8. This compound is toxic and occurs when powdered osmium is brought in touch with air. It is a water-soluble, pale yellow, highly volatile crystalline solid which has a strong odour. Powered osmium has a typical odour of osmium tetroxide, and when undergoing reaction with a base, it yields red osmates OsO4(OH)2-2. When reacting with NH3, it produces nitrito osmates OsO3N-. This compound’s boiling point is 130 degree Celsius and is a potent oxidising agent. Contrarily, osmium dioxide is a non-volatile, black, less toxic and much lesser reactive compound.
Only two compounds of osmium have vast applications:
Osmium Tetroxide: Used in electron microscopy for staining tissue and alkene oxidation in organic synthesis.
Non-volatile Osmates: Used in reactions of organic oxidation.
Synthesis of osmium trifluoride is not yet done, but osmium pentafluoride is a well-known compound. Larger halogens stabilise low oxidation states, such that tribromide, triiodide, tribromide and diiodide are known. Osmium iodide is known to exhibit +1 oxidation state, but some osmium carbonyl complexes show 0 oxidation state like triosmium dedocacarbonyl.
Generally, ligands which are suitable sigma donors (like amines) and pi acceptors (heterocycles consisting nitrogen) stabilise osmium’s low oxidation states. On the other hand, strong pi and sigma donors like N3- and O2- stabilise high oxidation states.
Although there is a wide range of compounds in multiple oxidation states, bulk osmium forms at standard pressure and temperature, which can resist attack from all acids (inclusive of aqua regia), even though, fused alkalis do attack osmium.
Other Chemical Properties:
Electronegativity – 2.2
Melting point – 3045 degree Celsius
Boiling point – 5027 degree Celsius
Van der Waals radius – 0.136 nm
Standard potential - + 0.85V
Now let us begin with the physical properties.
Osmium is two times denser than lead and a bit more dense than iridium.
It is a lustrous metal which shines at high temperatures too.
It has a high bulk modulus between 395 to 462 GPa.
This metal is quite hard and has low vapour pressure, and because of these reasons, solid osmium metal is hard to form.
The next section will give you an idea about the isotopes of osmium.
There are seven isotopes of osmium that occur naturally. Out of these seven, six are stable, and they are 184Os, 187Os, 188Os, 189Os, 190Os and 192Os. 186Os experiences alpha decay with a long half-life of 2.0 + 1.1 x 1015 years, which is almost 140,000 times of the Universe’s age. So it can be counted as stable for practical purposes. Prediction of alpha decay for all the seven isotopes is made, but only observation is done for 186Os. Furthermore, predictions also suggest that 184Os and 192Os can experience double beta decay.
Now that you have read about the isotopes, know more about osmium occurrence, applications, price and precautions in the subsequent sections.
Occurrence of Osmium
This metal is an even-numbered element, which puts itself in the top part of common elements found in space. In nature, osmium is found in natural alloys like:
Iridosmium (iridium rich)
Osmiridium (osmium rich)
In copper and nickel deposits, metals of platinum group exist as tellurides, sulphides, arsenides and antimonides. Platinum in all these four compounds is interchanged by a small quantity of osmium and iridium. Like all the platinum group metals, osmium is naturally available in alloys with copper or nickel.
In the Earth’s crust, like iridium, osmium metal is also found in largest concentrations in three kinds of geologic structures such as igneous deposits, impact craters and reworked deposits from any one of the former structure.
Did You Know?
Bushveld Igneous Complex in South Africa is the highest known primary reserve of osmium. Other two primary sources are large deposits of copper-nickel, which is closer to the Sudbury Basin in Canada and Norilsk in Russia. Moreover, some small reserves are found in the United States of America.
Another source of platinum group metals is the alluvial deposits in Colombia which were used by pre-Columbian citizens. The next alluvial deposit is in Russia’s Ural Mountains, which is mined till date.
Osmium in its pure form is rarely used; rather this element is alloyed with other metal elements for high-wear uses like:
Alloys of osmium like osmiridium are tough and with some other metals within the group of platinum are utilised to make fountain pen tips, electrical contacts and instrument pivots. These elements were used to make phonograph styli tips during the early “45” and “LP” record era. The tips made out of osmium alloys are more long-lasting than chromium and steel, and were expensive than diamond and sapphire tips. Therefore, their use was stopped.
Osmium tetroxide is used in fingerprint detection and fatty tissue staining for electron and optical microscopy.
Oslamp was constructed with an osmium made filament by Auer von Welsbach in 1898.
Tetroxide and potassium osmate (the formers derivate) are essential oxidants in organic synthesis.
Similar to palladium, osmium in powder form absorb atoms of hydrogen and may help in making metal-hydride battery electrode.
Osmium has a clinical use called synovectomy for patients who have arthritis, in Scandinavia.
In 2011, two compounds osmium (II) and osmium (VI) were reported showing anticancer activities in vivo. Hence, it suggested a hopeful future to use compounds of osmium as anticancer medicines.
Commonly, osmium metal is sold in the form of pure powder. Similar to other valuable metals, the unit of measure of osmium are grams and troy weight. As there is a minimal change in demand and supply of osmium, there is no alteration in the market price of osmium for decades. Moreover, since it is available in little amounts, working with this metal is challenging, has very fewer applications and it is difficult to keep it safe as it yields toxic compounds on oxidation.
The price of osmium per troy ounce has been the same from the 1990s. However, due to inflation, the metal has lost one-third of its price in the last two decades before 2019.
Safety Measures Related to Osmium
At room temperature, finely powdered osmium metal being pyrophoric, undergo reaction with oxygen and forms volatile osmium tetroxide. This produced compound can readily penetrate the skin and is harmful when inhaled, ingested or comes in contact with skin. Low concentrations of airborne osmium tetroxide vapour can lead to congestion in lung and skin, and damage the eyes. Hence, it must be used along with fume hoods.
Osmium tetroxide can be quickly reduced to nearly inert compounds using polyunsaturated vegetable oils or ascorbic acid.
Do It Yourself
1. What is the Boiling Point of Osmium?
a) 5027 degree Celsius
b) 5720 degree Celsius
c) 5207 degree Celsius
d) 5072 degree Celsius
2. Which of the Following can Attack Bulk Osmium?
d) aqua regia
3. Osmium Alloys are Mainly Used to Make:
a) fountain pen tips
If you wish to know more about osmium and its features and uses, please refer to our study materials. You can also download the Vedantu app and attend the online classes to learn about other metal elements and clear your doubts regarding the same.
1. Which Metal on Earth is the Heaviest One?
Ans. Iridium and osmium are the densest metals on Earth. However, in terms of relative atomic mass, plutonium and uranium are the heaviest metals.
2. How is Osmium Obtained Nowadays?
Ans. Osmium is obtained by processing nickel and platinum ores.
3. What is the Electronic Configuration of Osmium?
Ans. Osmium electronic configuration is Xe 4f14 5d6 6s2.
4. Which are the Rarest Metals on Earth?
Ans. Two metals, namely rhodium and osmium, are the rarest metals which are found on Earth.