rhenium

rhenium
Number	75
Name	rhenium
Category	transition metal
Standard atomic weight	186.207
Electron configuration	[Xe] 4f14 5d5 6s2
Phase	solid
Melting point degC	3186
Boiling point degC	5596
Density g cm3	21.02
Oxidation states	−3, −1, 0, +1, +2, +3, +4, +5, +6, +7
Crystal structure	hexagonal close-packed

rhenium. A rare, silvery-white transition metal, it is one of the densest elements and possesses the third-highest melting point of any element, after tungsten and carbon. Its discovery in 1925 by Walter Noddack, Ida Tacke, and Otto Berg filled one of the last gaps in the periodic table, and it was named for the Rhine River. Primarily obtained as a by-product of molybdenum refining from porphyry copper ores, rhenium's unique properties make it invaluable in high-temperature superalloys for jet engine components and as a catalyst in the petrochemical industry.

Properties

Rhenium is characterized by its exceptional density and refractory nature, with a melting point exceeded only by tungsten and carbon. It has a high modulus of elasticity and retains significant strength at elevated temperatures, resisting creep (deformation) better than most metals. The element exhibits a wide range of oxidation states, with the +7 state being particularly stable, as seen in compounds like rhenium(VII) oxide and potassium perrhenate. Its hexagonal close-packed crystal structure contributes to its mechanical stability, and it forms a volatile oxide, rhenium heptoxide, which can be readily reduced to the pure metal. Rhenium is also notable for its poor machinability and tendency to work-harden, requiring specialized processing techniques.

History

The existence of rhenium was predicted by Dmitri Mendeleev's periodic table, which indicated a gap below manganese in group 7. The element was discovered in 1925 by the German team of Walter Noddack, Ida Tacke, and Otto Berg while analyzing the mineral columbite using X-ray spectroscopy. They named it after the Rhine River (Rhenus in Latin). The first isolated gram quantities of the metal were produced in 1928 by Noddack and Tacke, who reduced potassium perrhenate with hydrogen gas. Commercial production remained negligible until the late 1950s and early 1960s, when demand surged for its use in platinum-rhenium catalysts for platforming in the petroleum refining industry and in high-temperature alloys for aerospace.

Occurrence and production

Rhenium is one of the rarest elements in the Earth's crust, with an average abundance comparable to ruthenium and rhodium. It does not occur as a native metal but is found in trace amounts, typically 0.001% to 0.2%, within certain molybdenite ores associated with porphyry copper deposits, such as those at the Chuquicamata mine in Chile and the Kennecott Utah Copper mine in the United States. The primary production method involves roasting molybdenum concentrates, during which rhenium volatilizes as rhenium heptoxide and is captured in scrubbers, yielding a solution of perrhenic acid. This is then processed into ammonium perrhenate and finally reduced with hydrogen in a tube furnace to produce a powdered metal, which is consolidated via powder metallurgy or arc melting.

Applications

The most significant use of rhenium is in nickel-based superalloys for high-temperature components in jet engines and gas turbines, such as turbine blades and combustion chambers, where it enhances creep (deformation) resistance. It is a critical component in platinum-rhenium catalysts used in catalytic reforming processes like platforming to produce high-octane gasoline. Other applications include filaments in mass spectrometers and ion gauges, electrical contacts in ignition systems for automobiles and aircraft, and heating elements in vacuum furnaces. The rhenium-188 isotope is used in nuclear medicine for radiotherapy of cancer, and rhenium compounds serve as catalysts in various organic synthesis reactions, including metathesis.

Biological role and precautions

Rhenium has no known biological role in any organism, including humans. Certain organorhenium compounds, such as those investigated by the Albert Einstein College of Medicine, have been studied for potential chemotherapy applications, though none are in clinical use. Soluble rhenium compounds, like potassium perrhenate, exhibit low to moderate toxicity in animal studies, primarily affecting the kidneys and liver if ingested in large quantities. The metal powder is a fire hazard, and its compounds should be handled to avoid inhalation or ingestion. Industrial exposure is regulated by agencies like the Occupational Safety and Health Administration, with guidelines similar to those for other heavy metals.

Isotopes

Naturally occurring rhenium consists of one stable isotope, rhenium-185, and one very long-lived radioactive isotope, rhenium-187, which undergoes beta decay to osmium-187 with a half-life of approximately 41.2 billion years. This decay system is the basis for the rhenium-osmium dating method, used extensively in geochronology to date iron meteorites, molybdenite ores, and the Earth's mantle. Artificial radioisotopes range from rhenium-160 to rhenium-194; among these, rhenium-186 and rhenium-188 are medically important. Rhenium-188, produced from a tungsten-188/rhenium-188 radioisotope generator, is used for palliative care in treating bone metastasis and in radioimmunotherapy.

Category:Chemical elements Category:Transition metals