Rhenium Facts – Element Re or Atomic Number 75

Rhenium (symbol Re and atomic number 75) is a dense silvery transition metal. It is one of the rarest metals and has the second highest melting point, after tungsten. Rhenium is important in jet engines, catalysts, and cancer therapies. Here is a collection of interesting rhenium facts..

Discovery, Naming, and Isolation

Credit for the discovery of rhenium often goes to German chemists Ida Noddack, Walter Noddack, and Otto Berg in 1925. While analyzing platinum ores and columbite, they identified rhenium and named it after the river Rhine (Rhenus in Latin), highlighting the element’s European roots. However, there is another earlier, lesser-known contributor to rhenium’s history—Japanese chemist Masataka Ogawa.

In 1908, Ogawa reported discovering a new element while studying thorianite (a rare thorium mineral) and initially named it nipponium (Np) after Japan. For decades, his discovery was controversial, partly because Ogawa had been seeking element 43, not 75. Later analysis of Ogawa’s work suggested that what he had identified as “nipponium” was likely rhenium, based on similarities in atomic weights and other chemical properties. Although Ogawa did not name rhenium or isolate it as a distinct element, his research offered an early indication of the presence of rhenium.

The Noddacks isolated rhenium and firmly established it as element 75. Initially, the isolation process was difficult due to the extremely low concentrations of rhenium in ores. Modern techniques make the extraction and purification of rhenium more efficient.

Appearance and Allotropes

Rhenium appears as a silvery-gray metal with a shiny, metallic luster. It crystallizes in the hexagonal close-packed crystal structure.

Characteristics and Key Properties

Rhenium possesses several notable characteristics:

• Density: Rhenium is extremely dense, with a density of approximately 21.02 g/cm³. The only elements with higher density values are osmium and iridium and possibly platinum.
• Melting Point: It has one of the highest melting points of any element at 3186°C (5767°F), surpassed only by tungsten and carbon.
• Boiling Point: Rhenium has one of the highest boiling points of the elements.
• Electrical Resistivity: Rhenium has a high electrical resistivity, which makes it useful in electronics. Its alloys become superconductors at low temperatures.
• Corrosion Resistance: It resists corrosion and oxidation at high temperatures.
• Catalytic Activity: Rhenium is highly effective as a catalyst, especially in petrochemical refining processes.

While forming rhenium typically requires sintering and pressure, the annealed metal is ductile.

Element Group

Rhenium is part of Group 7 in the periodic table, placing it alongside manganese and technetium. It is also in the d-block of transition metals, which have variable oxidation states and the ability to form complex compounds. Its rarity and corrosion resistance place it as a noble metal and precious metal.

Natural and Synthetic Isotopes

Natural rhenium consists of two isotopes:

1. Rhenium-185: Stable and makes up about 37.4% of natural rhenium.
2. Rhenium-187: Radioactive with a half-life of around 41.6 billion years, making it effectively stable for most practical purposes. It comprises about 62.6% of natural rhenium.

There are 33 synthetic radioisotopes of the element, ranging from rhenium-160 to rhenium-194. Rhenium-188 has medical applications due to its radioactive decay properties.

Abundance and Sources

Rhenium is a rare element with an estimated crustal abundance of only about 1 part per billion (ppb). Its extraction is as a by-product of molybdenum and copper mining. Rhenium occurs in minerals such as molybdenite (MoS₂), where it occurs in trace amounts. Major rhenium-producing countries include Chile, the United States, Poland, and Peru. The extraction of rhenium is challenging due to its low concentration and the complexity of separating it from other elements. Rhenium from used catalysts often gets recycled, making it one of the few metals where recycling is a significant source.

Purification

Rhenium extraction begins by roasting molybdenite, which converts the molybdenum sulfide in the ore into molybdenum oxide. This process releases rhenium as rhenium heptoxide (Re₂O₇), which then dissolves into water. Electrolysis or chemical reduction yields pure metallic rhenium from the solution. The purity of rhenium metal often exceeds 99.9%, making it suitable for high-performance applications.

Uses of Rhenium

The first commercial use of rhenium was in the manufacture of high-octane leaded gasoline. Today, rhenium has a variety of specialized applications:

1. Superalloys: Rhenium is a key element in nickel-based superalloys that improves their mechanical strength and resistance to heat. These superalloys find use in jet engines and gas turbines. Around 70% of rhenium production goes to jet engine parts.
2. Catalysts: Rhenium is an important catalyst in the petroleum industry, particularly in reforming processes that produce high-octane gasoline.
3. Thermocouples: Rhenium is important in high-temperature thermocouples, often paired with tungsten, for measuring extreme temperatures in furnaces and turbines.
4. Electrical Contacts: Due to its resistance to wear and corrosion, rhenium is useful in electrical contacts in high-performance electronic systems.
5. Filaments: Rhenium finds use in filaments for mass spectrometers, electron microscopes, and ion gauges due to its stability at high temperatures.
6. Medical Applications: Rhenium-188 is a radiopharmaceutical for cancer treatment.
7. Precision Instruments: The element finds use in precision equipment, such as gyroscopes. The durability, density, and mechanical stability of the metal ensure performance under demanding conditions.
8. High Pressure Applications: Due to its stiffness and high melting point, rhenium makes a good gasket for high pressure experiments.

Oxidation States

Rhenium exhibits multiple oxidation states, ranging from -3 to +7 (not -2). The most common and stable oxidation states are +3, +4, and +7.

Biological Role, Health Effects, and Toxicity

Like most extremely rare elements, rhenium has no known biological role in humans or other organisms. It is not an essential element and is essentially inert in biological systems. Exposure to rhenium compounds often poses health risks, but this relates more to other elements in the salts. Inhalation of rhenium dust irritates the respiratory system, as with other metals. Rhenium displays low toxicity compared against other heavy metals like lead or mercury.

Key Rhenium Facts for Scientists

References

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
• Noddack, W.; Tacke, I.; Berg, O. (1925). “Die Ekamangane”. Naturwissenschaften. 13 (26): 567–574. doi:10.1007/BF01558746
• Rouschias, George (1974). “Recent advances in the chemistry of rhenium”. Chemical Reviews. 74 (5): 531. doi:10.1021/cr60291a002
• Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4.
• Yoshihara, H. K. (2004). “Discovery of a new element ‘nipponiumʼ: re-evaluation of pioneering works of Masataka Ogawa and his son Eijiro Ogawa”. Spectrochimica Acta Part B: Atomic Spectroscopy. 59 (8): 1305–1310. doi:10.1016/j.sab.2003.12.027