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Osmium

physical science Maturity 7-9 Next article →

Osmium is a rare and remarkable chemical element. It is represented by the symbol Os and has the atomic number 76. As a member of the platinum group, it is a transition metal.

Osmium 1-crop
Osmium 1-crop

Osmium is characterized by its bluish-white color and its extreme physical toughness. It is a hard but brittle metal, meaning it can break easily if struck. One of its most defining features is its density. Osmium is the densest stable element in existence. It is approximately twice as dense as lead. This extreme density makes it a unique subject for scientific study.

The physical properties of osmium are quite extraordinary. It has a very low compressibility, which means it does not squash easily under pressure. Because of this, its bulk modulus is extremely high. The bulk modulus is a measure of how much a substance resists compression. Osmium's value in this area rivals that of diamond. The metal also has a very high melting point. In fact, it has the fourth highest melting point of all elements, following carbon, tungsten, and rhenium.

Osmium cluster
Osmium cluster

These traits make osmium very difficult to machine, form, or work into specific shapes.

Osmium is chemically versatile and can exist in many different oxidation states. An oxidation state describes the charge an atom carries when it forms compounds. Osmium can range from a charge of −2 to +8. The most common states are +2, +3, +4, and +8. The +8 state is particularly significant. It is one of the highest oxidation states reached by any element.

Osmiumtetroxide1
Osmiumtetroxide1

A common compound of osmium is osmium tetroxide, which has a +8 state. This compound is a volatile, water-soluble solid. It is known for having a pronounced and nauseating smell. Because of this scent, the element was named after the Greek word "osme," meaning smell.

The history of osmium is tied to the study of platinum. In the late 17th century, platinum was found in silver mines in Colombia. By 1748, chemists realized platinum was a distinct element. When scientists dissolved platinum in aqua regia, a mixture of acids, a dark residue remained. Some thought this residue was graphite. However, in 1803, Smithson Tennant and William Hyde Wollaston investigated this residue in London. Tennant discovered that the black powder actually contained two new metals: iridium and osmium.

Platinum nuggets
Platinum nuggets

He documented these findings in a letter to the Royal Society in 1804.

Osmium is very rare in the Earth's crust. Its average mass fraction is only 50 parts per trillion in the continental crust. It is often found in nature as alloys, such as osmiridium or iridosmium.

Platinum nuggets
Platinum nuggets

Most osmium is obtained as a byproduct of mining nickel and copper. During the electrorefining of these metals, precious metals settle at the bottom as "anode mud." Scientists then use various chemical processes to separate the osmium. They might use fusion with sodium peroxide or distillation of osmium tetroxide. Annual worldwide production is estimated to be between several hundred and a few thousand kilograms.

Scientists use different versions of osmium, called isotopes, to learn about the past. There are seven naturally occurring isotopes of osmium. Five of these are stable. One specific isotope, Os-187, is very important for geological dating. It is the descendant of Rhenium-187. This relationship allows scientists to use rhenium-osmium dating. This method helps determine the age of rocks on Earth and meteoric rocks from space. It can even help researchers understand the history of the Earth's mantle.

Beyond geology, osmium has played a role in industrial chemistry. In the past, uranium and osmium were used as catalysts in the Haber process. This process combines nitrogen and hydrogen to produce ammonia. Using osmium made the process economically successful for a time. However, in 1908, cheaper iron-based catalysts were introduced. This change reduced the need for expensive osmium. Today, osmium is primarily used in specialized alloys. These alloys are used for fountain pen nib tips and electrical contacts because they require extreme durability and hardness.

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File:Osmium 1-crop.jpg
Osmium 1-crop.jpg
File:Osmiumtetroxide1.jpg
Osmiumtetroxide1.jpg
File:Platinum nuggets.jpg
Platinum nuggets.jpg
File:Osmium cluster.jpg
Osmium cluster.jpg
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