Rhenium

Rhenium (Re)

Introduction

Rhenium is a chemical element with the atomic number 75 in the periodic table. It’s one of the rarest elements found in Earth’s crust. Being a member of the transition metals group of elements, this rare metal element has seven valence electrons that make it a chemically inert substance to most fuel gases except oxygen. Its chemical compounds can occur in all oxidation states from -1 to +7.

Element 75 is the last one to be discovered in nature, and is mainly used for its high temperature, corrosion, and wear properties. In medicine, isotopes of rhenium are used in experimental therapies administered to cancer patients. For this purpose, medical radioactive rhenium-amino compound phosphate is typically used. 

Fact Box

Chemical and Physical Properties of Rhenium

The symbol in the periodic table of elements: Rh

Atomic number: 75

Atomic weight (mass): 

Group number: 7 (Transition metals)

Period: 6 (d-block)

Color: A silvery-white metal

Physical state: Solid at 20°C

Half-life: From 860(120) milliseconds  [0.82(+15−9) milliseconds] to 4.1× 1010years

Electronegativity according to Pauling: 1.9

Density: 20.8 g. cm-3

Melting point: 3185°C, 5765°F, 3458 K

Boiling point: 5590°C, 10094°F, 5863 K

Van der Waals radius: 

Ionic radius: .53 (+7) Å

Isotopes: 34

Most characteristic isotope: 187Re

Electronic shell: [Xe] 4f145d56s2

The energy of the first ionization: 759 kJ.mol-1

The energy of the second ionization: N/A 

Discovery date: In 1925 by Walter Noddack, Ida Tacke, and Otto Berg

Rhenium’s properties were predicted by the great Russian chemist Dmitri Mendeleev even since he conceptualized the periodic system of elements. Classified in the periodic table under the symbol Re, atomic number 75, atomic mass of X g.mol-1, and electron configuration [Xe]4f145d56s2, rhenium is an extremely hard, and very dense silvery-white metal. It reaches its boiling point at 5590°C, 10094°F, 5863 K, while its very high melting point is achieved at 3185°C, 5765°F, 3458 K. 

This member of the transition metals family of elements has an electronegativity of 1.9 according to Pauling, whereas the atomic radius according to van der Waals is 0.138 nm. Similar to elements molybdenum, niobium, tantalum, and tungsten, rhenium also has excellent refractory, anti-corrosive, and anti-oxidative properties, as well as good wear resistance. It also has one of the highest melting points among the elements of the periodic table, which makes rhenium very resistant to heat.                         

How Was Rhenium Discovered?

When the Russian chemist Dmitri Mendeleev projected the periodic system of elements, many of the chemical elements we know today were undiscovered. And these empty slots are exactly what Mendeleev based the periodic table on at the time. 

Namely, by classifying the chemicals into groups (families) and periods, he was considering the similarities of both physical and chemical properties of the elements. Two among the elements Mendeleev predicted were the ‘eka-manganese’ (technetium), and ‘dvi-manganese’ (rhenium). However, they were discovered much later. 

The Contribution of Henry Mosely 

In 1913, the English physicist Henry Moseley (1887-1915) analyzed Mendeleev’s concept and concluded that it would make more sense if the chemical elements are arranged by their atomic numbers, not by the atomic weight. Moseley’s concept brought the undiscovered elements even closer to their right places in the periodic table and eased the work on the future discoverers of elements. The scientists now were clearly directed to the discovery of element 75 by knowing the projected properties of the chemical that was yet to be discovered.

The Discovery of the German Team of Scientists

Following the guidelines set by Mendeleev and Mosley, the German chemists Walter Noddack, Ida Tacke, and Otto Berg, attempted to discover the substance that was projected to fill in the gap of element 75 of the periodic table. For this reason, they processed 660 kg of molybdenite ore for producing only one gram of rhenium. 

How Did Rhenium Get Its Name?

Element 75 was named ‘rhenium’ after the Latin name of the river Rhine, ‘Rhenus’. The river was flowing near the laboratory where Walter Noddack, Ida Tacke, and Otto Berg succeeded in isolating element 75. 

Where Can You Find Rhenium?

Element 75 is labeled as a rare metal since it’s always found combined with other elements. Also, there are no mineable ores from which rhenium can be extracted. Rhenium often occurs as an impurity in platinum ore, molybdenum, and copper ores, as well as in the minerals gadolinite, and columbite.

Rhenium occurs up to about 20 parts per million in molybdenite and to a lesser extent in sulfidic copper ores. Commercial rhenium is often obtained as a by-product of the purification process of molybdenum and copper-sulfide ores, or by extraction from the flue dusts of molybdenum smelters. The refining process includes smelting of the molybdenite ore, electrolysis of copper, as well as hydrogen reduction to obtain rhenium metal. 

Cold-working and annealing of the alloys are the preferred methods in the production of rhenium wire and foil. Element 75 can also be consolidated by pressing and sintering of this substance in a vacuum or hydrogen atmosphere.

The richest rhenium deposits are found in Chile, followed by the United States (New Mexico, Utah, Arizona, and Colorado), Russia, Poland, Uzbekistan, and Kazakhstan.

Rheniite

Discovered at the Kudriavy Volcano, Iturup Island in the Kuril Islands, Russia, rheniite is the first rhenium mineral that has been discovered. It’s also found in northeastern Greece in the Pagoni Rachi Mo–Cu–Te–Ag–Au deposit, where it occurs along with molybdenite in quartz veins.

Rhenium in Everyday Life

Rhenium applications are versatile, mainly due to the high refractory properties of this transition metal: 

  • Rhenium alloys are applied in the production of X-ray anodes and X-ray tubes;
  • The metal form of rhenium has versatile uses. It can be utilised in the manufacturing of many products, among which we find jewelry, ovens, electrodes, thermocouples, electrical contacts, igniters for flash bulbs, plating, non-deformable heating elements, electrical contact material, as a filament of an X-ray target, heater for fast start of color TV, etc.
  • Petroleum refineries use rhenium catalysts together with platinum in the production of a lead-free and high-octane gasoline;
  • About 6% of rhenium is added to nickel alloys for the production of single-crystal turbine blades and exhaust nozzles of jet engines;
  • As a widely used superalloy, rhenium is added to the alloys in jet engines and gas turbines to support their stability when exposed to high temperatures, as well as to provide higher protection of the metals from corrosion;
  • Being one of the refractory metals, rhenium and rhenium alloys are used for manufacturing of high-temperature and high-strength parts of supersonic aircraft and missiles;
  • Nowadays, rhenium matrix composite material is often used as the base material in the ultra-high temperature emitters;
  • At temperature of 10K, rhenium-molybdenum alloys act as superconductors;
  • Sodium perrhenate and ammonium perrhenate are the two most commercially exploited rhenium compounds. They are used in 
  • The Re-186 and Re-188 isotopes of this chemical element have a significant application in the field of radiology. They are included in liver cancer therapy (186Re), as well as in the experimental treatment of malignant forms of cancer, such as pancreatic cancer (188Re). 

How Dangerous Is Rhenium?

Despite being generally considered as a substance of low toxicity, inhalation of the metal dust of this chemical element may lead to irritation of the lungs and difficult breathing. If the metal dust particles come into contact with the eyes or skin, they may trigger irritation, redness, and burns. In case of prolonged chronic exposure to rhenium, dizziness, headaches, and suffocation may occur.

When the properties of rhenium trichloride and potassium perrhenate were studied on rats, they displayed very low toxicity, similar to the harmfulness of table salt (sodium chloride). If element 75 is exposed to high temperatures, it forms the highly explosive yellow heptoxide powder (Re2O7) that poses both a risk of explosion and presents a fire hazard. 

Environmental Effects of Rhenium

Since the rhenium quantity in nature is extremely low, there isn’t sufficient evidence of its hazardous effects upon the environmental systems. 

Isotopes of Rhenium

There are 34 isotopes of rhenium, among which only one is stable. Naturally occurring rhenium is formed by two of its isotopes: rhenium-187 (62.6% abundance) and rhenium-185 (37.4% abundance). With a half-life of 70 days, 183Re is the longest living form of element 75. The highly radioactive rhenium-187 isotope has an abundance of 62.6 percent, and a half-life of 4.1× 1010years.

Main Isotopes of Rhenium (75Re)

Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
185Re 37.4% stable
187Re 62.6% 4.12×1010 y β 187Os

Source: Wikipedia

List of Rhenium Compounds 

As a part of a chemical compound, rhenium adopts various oxidation states, but most frequently it occurs in +2, + 4, +6, and +7 oxidation states. The rhenium metal typically forms oxides, oxyhalides, sulfides, as well as halides. 

 

  • Oxotrichlorobis(triphenylphosphine)rhenium(V)
  • Potassium nonahydridorhenate
  • Potassium octachlorodirhenate
  • Rhenium diboride
  • Rhenium diselenide
  • Rhenium disulfide
  • Rhenium ditelluride
  • Rhenium fluoride
  • Rhenium heptafluoride
  • Rhenium hexafluoride
  • Rhenium pentachloride
  • Trirhenium nonachloride
  • Rhenium trioxide
  • Rhenium(IV) chloride
  • Rhenium(IV) oxide
  • Rhenium(VI) chloride
  • Rhenium(VII) oxide
  • Rhenium(VII) sulfide
  • Ammonium perrhenate
  • Perrhenic acid
  • Silver perrhenate
  • Sodium perrhenate

5 Interesting Facts and Explanations

  1. Rhenium is the fourth densest chemical element, following platinum, iridium and osmium, as well as the third element with the highest melting point, after the chemicals tungsten and carbon.
  2. Transcribed from Sanskrit, ‘eka’ has the meaning of ‘first’, while ‘dvi’ means second. Despite the fact that rhenium was labeled as ‘dvi-manganese’ and technetium ‘eka-manganese’, element 75 (Re) was discovered before element 43 (Tc). 
  3. Rhenium diboride is one of the hardest known substances in the world. The hardness of this rhenium compound can be compared to the one of zirconium diboride, silicon carbide, or titanium diboride. 
  4. Rhenium was the last naturally occurring chemical element that has been discovered with at least one stable isotope. Element 75 filled the last empty slot of Mendeleev’s predicted periodic table. The rest of the elements discovered after rhenium (up to element 118) are synthetically produced, and cannot be found in Earth’s crust.
  5. Japanese chemist Masataka Ogawa (1865-1930) claimed to have discovered the final element of Mendeleev’s periodic table in a rare thorium oxide mineral labeled as thorianite in 1908. He assigned it as element 43 instead of 75 and named it ‘nipponium’. Unfortunately, no other scientist at that time could confirm his discovery which led them to the conclusion that Ogawa might have wrongly assessed the new element’s properties.