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

It wasn't until 1998 that I took a real look at element 43. I was now at the National Institute of Standards & Technology (NIST), in the Surface & Microanalysis Science Division. One day, an exuberant Belgian physicist, Pieter van Assche, came into my office to ask my interpretation of an X-ray emission spectrum. The spectrum was from a 1925 article by Ida Noddack-Tacke, Walter Noddack, and Otto Berg, who claimed to have discovered element 43 (which they named "masurium") in samples from uranium-rich ores.

Van Assche speculated that, although the researchers didn't realize it, they had isolated terrestrial technetium-99 formed from the spontaneous fission of uranium. I was skeptical, but after studying their paper, I realized that they were clearly not crackpots or, as Ernest Lawrence called them, "apparently deluded." In the same article, the authors claimed discovery of element 75, naming it "rhenium." Both claims were widely disputed at the time, but three years later, the Noddacks isolated weighable amounts of rhenium and were accepted as its discoverers. They weren't able to so concentrate masurium, and the International Union of Pure & Applied Chemistry eventually rejected that discovery. The controversy clearly affected their reputations.

Little attention was paid to Ida Noddack-Tacke's article in 1935 questioning Enrico Fermi's claim that he discovered the transuranium element 93 (for which he received the Nobel Prize) and suggesting that his neutron bombardment of uranium may have resulted in the atoms disintegrating into fragments. It was not until Lise Meitner and colleagues' "discovery" of nuclear fission in 1939 that she was proved right. After this time, the Noddacks led lives of relative scientific obscurity.

Using first-principles X-ray-emission spectral-generation algorithms developed at NIST, I simulated the X-ray spectra that would be expected for Van Assche's initial estimates of the Noddacks' residue compositions. The first results were surprisingly close to their published spectrum! Over the next couple of years, we refined our reconstruction of their analytical methods and performed more sophisticated simulations. The agreement between simulated and reported spectra improved further. Our calculation of the amount of element 43 required to produce their spectrum is very similar to the direct measurements of natural technetium abundance in uranium ore published in 1999 by Dave Curtis and colleagues at Los Alamos. We can find no other plausible explanation for the Noddacks' data than that they did indeed detect fission "masurium."

The Noddacks were clearly among the finest analytical geochemists of their time. Their search for the "missing" elements below manganese in the periodic table was part of a larger effort to accurately determine the abundance of the chemical elements in the earth and meteorites--data that provided a foundation for the science of geochemistry. Their work complemented rather than detracted from that of Perrier and Segrè.

I am pleased to see that Ida Noddack-Tacke's contributions to science are being rediscovered--on the Web and in recent books about the periodic table. As a participant in this scientific detective adventure, I'll always have a fondness for the "element that was discovered twice"--first as masurium, the first natural element discovered composed entirely of a spontaneous fission product; second as technetium, the first man-made chemical element.

John T. Armstrong is a research chemist at NIST, Gaithersburg, Md. He does fundamental and applied research on electron and X-ray spectrometry and is a past president of the Microbeam Analysis Society.

TECHNETIUM AT A GLANCE

Name: From the Greek technetos, artificial.

Atomic mass: (98).

History: Discovered in 1937 by Carlo Perrier and Emilio Segrè. It is the first artificially produced element. Since its discovery, searches for technetium in terrestrial materials have been made without success until recently. Technetium has been found in the spectrum of S-, M-, and N-type stars, and its presence in stellar matter is leading to new theories of the production of heavy elements in the stars.

Occurrence: Artificially produced.

Appearance: Silvery gray metal.

Behavior: Radioactive. Tarnishes slowly in moist air.

Uses: Used as a medical tracer and to calibrate particle detectors.

RHENIUM

FRED BROT, SIGMA-ALDRICH

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.