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The missing link" is a term used in a wide variety of ways. While it originated with anthropologists, these days it can be applied to genetics, movies, social programs, Web links, or even just general conversation when something is noticeably absent.

If you think about it in terms of chemistry, the periodic table had a missing link until 1925. It was the element rhenium.

Rhenium, element 75 on the periodic table, belongs to the cluster of elements known as the transition group of elements. It was the last of the natural elements to be discovered, some 50 years after the introduction of the periodic table. It is a silvery white, rare, heavy, polyvalent transition metal. Chemically, it resembles manganese and is used in some alloys. Rhenium has one of the highest melting points of all elements, exceeded only by tungsten and carbon. It is also one of the most dense, exceeded only by platinum, iridium, and osmium.

Rhenium's usual commercial form is a powder, but this element can be consolidated by pressing and resistance-sintering in a vacuum or hydrogen atmosphere. This procedure yields a compact shape that is in excess of 90% of the density of the metal. When annealed, this metal is very ductile and can be bent, coiled, or rolled. Rhenium-molybdenum alloys are superconductive at 10 K.

Rhenium was discovered in 1925 by Walter Noddack, Ida Noddack-Tacke, and Otto Berg in Germany in platinum ores, such as columbite and tung-state. J. G. F. Druce discovered it independently in manganese sulfate. The ores from which rhenium was first isolated commercially came from the region of the Rhine River; hence the element's (modest) name derives from the Latin term for the river, Rhenus.

BIRTHPLACE Rhenium was first isolated commercially from ores found near the Rhine River.
Rhenium possesses a hexagonal, close-packed crystal structure. It has the electronic structure [Xe]4f14 5d5 6s2, a melting point of 3,452.2 K, a boiling point of 5,923 K, an electronegativity of 1.9 (Pauling), and a thermal conductivity of 48 J per meter per second per kelvin.

Natural rhenium is a mixture of one stable and one radioactive isotope of very long half-life, although most sources report two stable isotopes. Twenty-six other unstable isotopes are recognized.

Rhenium does not occur free in nature or as a compound in a particular type of mineral. It is widely spread throughout Earth's crust at approximately 1 to 4 ppb. Commercially, rhenium is obtained through the processing of copper-sulfide ores that contain molybdenum. In this purification, the molybdenum occurs as a sulfurous sludge, which, at elevated temperatures, releases rhenium. The next step in the process is production of the ammonium salt ammonium perrhenate. APR is the product sold to metal brokers and catalyst manufacturers. Rhenium is produced by reducing APR with hydrogen.

Applications for rhenium are numerous. It is used in filaments for mass spectrographs and ion gauges; in electrical contact material, as it has good wear resistance and withstands arc corrosion; in thermocouples (those made of rhenium-tungsten are used for measuring temperatures to 2,200 °C); in wire used in flash lamps for photography; and in additives to tungsten and molybdenum-based alloys to increase ductility at higher temperatures. Because of rhenium's high resistance to poisoning from nitrogen, sulfur, and phosphorus, rhenium catalysts are used for the hydrogenation of fine chemicals and the disproportionation of alkenes.

What's clear is that while rhenium may once have been missing, it's difficult to imagine the periodic table today without it!

Fred Brot is a technical service scientist for Sigma-Aldrich. He also serves as a technical writer and editor for scientific documents.


Chemical & Engineering News
Copyright © 2003 American Chemical Society

Name: From the Greek Rhenus, Rhine, a major European river.
Atomic mass: 186.21.
History: Discovered in 1925 by German chemists Walter Noddack, Ida Noddack-Tacke, and Otto C. Berg.
Occurrence: Does not occur in nature as a free metal. The minerals gadolinite and molybdenite contain small quantities.
Appearance: Silvery white with a metallic luster.
Behavior: Tarnishes slowly in moist air. Rhenium does not react with water under normal conditions. Annealed rhenium is very ductile and can be bent, coiled, or rolled. The metal dust is a fire and explosion hazard.
Uses: Used in filaments for mass spectrographs, thermistors, and catalysts and as an additive to tungsten- and molybdenum-based alloys. Rhenium wire is used in photoflash lamps. Rhenium is also used as an electrical contact material because it has good wear resistance and withstands arc corrosion. Rhenium catalysts are exceptionally resistant to poisoning from nitrogen, sulfur, and phosphorus and are used for the hydrogenation of fine chemicals, hydrocracking, reforming, and the disproportionation of alkenes.

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