Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

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.

RHENIUM AT A GLANCE

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.

BOHRIUM & HASSIUM

PETER ARMBRUSTER, GESELLSCHAFT FÜR SCHWERIONENFORSCHUNG

Nearly a century ago, Ernest Rutherford showed that the nucleus of an atom has identical proton and atomic numbers (Z). The stability of the nucleus and the number of possible elements then became a question of nuclear physics. The ratio of two forces--the short-range nuclear force and the long-range electromagnetic force--determines the stability of nuclei. The nuclear force keeps nucleons together, whereas electric repulsion between protons tries to break the nucleus into smaller pieces. Small changes in the ratio of the two forces decide whether a chemical element can or cannot exist.

As Z increases, the electric repulsion between protons rises in proportion to the square of their number, whereas the attractive nuclear forces grow less than linearly with the total number of nucleons. An energy barrier protects the atomic nucleus against fission. This barrier becomes smaller and smaller as Z increases. Danish physicist Niels Bohr predicted in 1939 that, assuming the nucleus to be a droplet of nuclear matter, the number of elements should be limited to about a hundred.

However, the quantum mechanical order of atomic electrons--the essence of chemistry--has an equivalent in atomic nuclei. Nuclear structure, as the order in the chaotic soup of nucleons is called, gives additional binding energy compared to a structureless nuclear droplet, and an increase in the number of possible chemical elements was predicted in the 1960s. This idea of new superheavy elements in the range up to Z = 120 stabilized by nuclear structure inspired nuclear researchers and, in Germany, led to different initiatives for entering the element-hunting race.

In December 1969, the Gesellschaft für Schwerionenforschung (GSI) was founded at Darmstadt in order to build a heavy-ion accelerator and start research on the physics and chemistry of superheavy elements. This decision led to the synthesis of six new elements between 1981 (Z = 107) and 1996 (Z = 112). Their atomic nuclei are strongly stabilized by the quantum-mechanical order of their constituents, and they have a barrel-like shape. These elements are the first superheavy elements. Their nuclei are protected against spontaneous fission decay by a high fission barrier built up by the nuclear structure of the system.

Our surprising success was a consequence of long-term planning combined with fortuitous circumstances. At the start of the project, we had a unique technological base in Germany. Christoph Schmelzer had started work in the late 1950s on acceleration of heavy ions, and Heinz Ewald and I developed and built recoil separators for fission fragments at nuclear reactors. These were essential provisions for an accelerator (UNILAC) and a recoil separator for fusion products (SHIP) to be available by 1975. Both of these--viewed somewhat skeptically by the outside community--were genuine innovations. UNILAC was built by the GSI team, and SHIP was designed and built in collaboration with the University of Giessen by a team headed by Gottfried Münzenberg. Besides the recoil-separator technique, new target technology and position-sensitive silicon-detector techniques were decisive. The successful team of Münzenberg, Sigurd Hofmann, Fritz Peter Hessberger, Willibrord Reisdorf and Karl-Heinz Schmidt synthesized elements Z = 107­109 between 1976 and 1989.

HEAVY RESEARCH Armbruster (center, standing) poses with his GSI colleagues. The group helped to discover six transuranium elements.