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Rhenium

physical science Maturity 7-9 Next article →

Rhenium is a heavy, silvery-gray transition metal with the chemical symbol Re. It is located in group 7 of the periodic table and has the atomic number 75. This element is exceptionally rare in the Earth's crust. Scientists estimate its average concentration is only 1 part per billion (ppb). Because it is so scarce, rhenium is quite expensive. In 2008 and 2009, its price reached an all-time high of US$10,600 per kilogram.

Molybdenit 1
Molybdenit 1

Physically, rhenium is a remarkable substance with extreme properties. It possesses one of the highest melting points of all elements. Only tungsten has a higher melting point. It also has one of the highest boiling points among all elements. Among the stable elements, its boiling point is the highest. Rhenium is also one of the densest elements known. It is exceeded in density only by platinum, iridium, and osmium.

Ammonium perrhenate
Ammonium perrhenate

Most rhenium is obtained as a by-product of mining other metals. It is primarily extracted during the refinement of molybdenum and copper ores. One major source is the mineral molybdenite, which is mostly molybdenum disulfide. In some ores, rhenium concentrations range from 0.001% to 0.2%. To extract it, the ore is roasted to volatilize rhenium oxides. These oxides, such as rhenium(VII) oxide, dissolve easily in water. They are then leached from flue dusts and purified into salts like ammonium perrhenate.

Nonahydridorhenate-3D-balls
Nonahydridorhenate-3D-balls

The history of rhenium is a story of mistaken identity. In 1908, Japanese chemist Masataka Ogawa announced he found a new element. He named it nipponium and thought it was element 43. In reality, he had discovered element 75. Because he was searching for element 43, he did not realize his mistake. Later, in 1925, a team in Germany rediscovered the element. Walter Noddack, Ida Tacke, and Otto Berg identified it correctly. They named it rhenium after the Rhine River in Europe.

Perrhenic-acid-3D-balls
Perrhenic-acid-3D-balls

Today, the most significant use for rhenium is in the aerospace industry. It is used to create nickel-based superalloys. These alloys can withstand incredible heat without losing strength. They are used for turbine blades, combustion chambers, and exhaust nozzles in jet engines. These alloys often contain up to 6% rhenium. This makes jet engine construction the largest single use for the metal.

CFM56 P1220759
CFM56 P1220759

Another vital role for rhenium is as a chemical catalyst. A catalyst is a substance that speeds up a chemical reaction. Rhenium is an excellent catalyst for hydrogenation and isomerization. One specific use is the catalytic reforming of naphtha to make gasoline. This is known as the rheniforming process. While demand for these catalysts has dropped recently, the metal remains essential for fuel production.

Pratt & Whitney F100 F-15 engine undergoing testing
Pratt & Whitney F100 F-15 engine undergoing testing

In the laboratory, rhenium shows complex chemical behavior. It can exist in many oxidation states, ranging from −3 to +7. This means it can gain or lose different numbers of electrons when forming compounds. Common states include +7, +4, and +3. It also has a unique isotopic makeup. Naturally occurring rhenium consists of 37.4% rhenium-185 and 62.6% rhenium-187. The rhenium-187 isotope is unstable, but it has a very long half-life of 41.6 billion years. Scientists use the decay of this isotope for rhenium–osmium dating of ores.

Perrhenic-acid-3D-balls
Perrhenic-acid-3D-balls
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File:Perrhenic-acid-3D-balls.png
Perrhenic-acid-3D-balls.png
File:Nonahydridorhenate-3D-balls.png
Nonahydridorhenate-3D-balls.png
File:Molybdenit 1.jpg
Molybdenit 1.jpg
File:Ammonium perrhenate.jpg
Ammonium perrhenate.jpg
File:Pratt & Whitney F100 F-15 engine undergoing testing.jpg
Pratt & Whitney F100 F-15 engine...
File:CFM56 P1220759.jpg
CFM56 P1220759.jpg
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