Rhenium

Rhenium is a very rare metal that was discovered in 1925 by Otto Berg, Ida Noddack and Walter Noddack. It has several distinct characteristics and is widely used in making of superalloys.

History and Discovery

Rhenium was discovered in 1925 by Otto Berg, Ida Noddack and Walter Noddack from ores of platinum and in various minerals. They also succeeded in extracting pure form of rhenium in 1928. The element was named Rhenium after a river in Europe, called Rhine. In order of discovery among elements, rhenium was the second last stable element [1].

Rhenium

Periodic Table ClassificationGroup 7
Period 6
State at 20CSolid
ColorSilvery-grayish
Electron Configuration[Xe] 4f14 5d5 6s2
Electron Number75
Proton Number75
Electron Shell2, 8, 18, 32, 13, 2
Density21.04 g.cm-3 at 20°C
Atomic number75
Atomic Mass186.21 g.mol -1
Electronegativity according to Pauling1.90

Occurrence

Rhenium is extremely rare and is considered as the rarest of all metals. it is found in about 0.5 ppb of Earth’s crust and is ranked as the 77th most abundant element in the Earth’s crust [2]. Rhenium does not exist in native form. The ores and minerals of rhenium are also quite and molybdenite and gadolinite are some of the minerals that contain trace amount rhenium. Rhenium is an expensive metal as it has a high demand and low availability. The largest producer of rhenium in the world is Chile, where the largest copper ore deposits of rhenium are found. Recently, a mineral of rhenium, rheniite (ReS2), is being used for extracting large amount of rhenium (20-60 kg rhenium per year) [3]. Other countries that are leading producers of rhenium include Peru, USA and Poland.

Physical Characteristics

Rhenium is a greyish silver metal. Rhenium has remarkably high boiling point (5627 °C) and is categorized as the element with the highest boiling point. It also has one of the highest melting point (3170 °C), second only to carbon and tungsten. Rhenium is a significantly dense metal (21.02 g/cm3). Rhenium do not dissolve in acids and alkalis at room temperature. It does not dissolve in aqua regia.  Rhenium is resistant to tarnishing and is corroded very slowly in the presence of moist air. The atomic number of rhenium is 75 and its atomic mass is 186.

Chemical Characteristics

Rhenium is not very reactive metal. It exists in many oxidation states in compounds, and +2, +4, +6 and +7 are the most common [4]. Rhenium is used in powder form. Rhenium is unreactive with water at standard conditions. Some salts of rhenium have commercial importance, such as sodium perrhenates and ammonium perrhenates. Rhenium forms oxide and the most common is rhenium (VII) oxide, which is a colorless and volatile compound. Rhenium also from various organic compounds, such as dirhenium decacarbonyl, methyl-rhenium trioxide and pentacarbonylhydridorhenium.

Significance and Uses

  • Rhenium is a widely used super alloy. It is used to make alloys with various metals to impart desirable characteristics. Rhenium is widely used to make high-temperature superalloys, that are used for manufacturing of parts of military jet engines.
  • Rhenium is used as catalyst in gasoline, in combination with platinum.
  • Rhenium superalloys are used in gas turbine engines in various industries.
  • Rhenium is used in making electrical contact points, mass spectrometers and photographic flash lamps.
  • Rhenium tungsten superalloys are used in making X-ray machines.
  • Rhenium isotopes (rhenium-186 and 188) are used to treat liver and pancreatic cancer (radio-pharmacy).

Health Hazards

Rhenium is a toxic metal. Exposure to rhenium can lead to irritation of eyes and skin. While, inhalation and ingestion of rhenium can lead to disturbance in respiratory and digestive tract, respectively. The vapors of rhenium cause breathing difficulty and dizziness. Certain compounds of rhenium, such as rhenium trichloride are highly toxic.

Isotopes of Rhenium

There are 25 radioactive isotopes of rhenium. Rhenium has only one stable isotope, rhenium-185. Rhenium-185 is quite rare, as natural rhenium contain about 37.4 % of rhemium-185 and about 62.6% of rhenium-187. Rhenium-187 is not highly unstable as it has a half-life of about 1010  years.

REFERENCES

[1]. “Rhenium: Statistics and Information”. Minerals Information. United States Geological Survey. 2011. Retrieved 2011-05-25

[2]. Emsley, John (2001). “Rhenium”. Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford, England, UK: Oxford University Press. pp. 358–360. ISBN 0-19-850340-7.

[3]. Tessalina, S.; Yudovskaya, M.; Chaplygin, I.; Birck, J.; Capmas, F. (2008). “Sources of unique rhenium enrichment in fumaroles and sulphides at Kudryavy volcano”. Geochimica et Cosmochimica Acta. 72 (3): 889. Bibcode:2008GeCoA..72..889T. doi:10.1016/j.gca.2007.11.015.

[4]. Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1985). “Rhenium”. Lehrbuch der Anorganischen Chemie (in German) (91–100 ed.). Walter de Gruyter. pp. 1118–1123. ISBN 3-11-007511-3.