Segrè Identifies the First Artificial Element

Date January-September, 1937

Emilio Gino Segrè positively identified and characterized the first human-made chemical element, technetium, atomic number 43.

Locale Palermo, Italy

Key Figures

• Emilio Gino Segrè (1905-1989), Italian-born American physicist
• Enrico Fermi (1901-1954), Italian physicist
• Ernest Orlando Lawrence (1901-1958), American physicist

Summary of Event

The periodic table of chemical elements is an ordered array of the elements positioned to reflect the similarities and trends among the different discrete substances that compose matter. When the table was first created in the mid-1800’s, controversy arose regarding placement of the limited number of chemical elements known at that time. The Russian chemistDmitry Ivanovich Mendeleyev is credited with the foresight to leave voids within the framework of his representation of the periodic table, predicting that as-yet-undiscovered elements would fill these voids. In the years following his prediction, the majority of these missing elements were discovered and properly placed within the framework of the modern periodic table, with some notable exceptions. Even after scientists gained a more thorough understanding of atomic structure, as developed in the early 1900’s, and with the work of Henry Moseley, who utilized X-ray spectral data to ascertain the atomic numbers of the elements available to him—thus providing experimental support to the periodic array previously based solely on similarities of chemical and physical properties and trends—voids remained.

The periodic table is arranged in order of increasing atomic number—that is, the number of protons within the nucleus of each atom of an element. There were no reports of discovery in the scientific literature on the isolation and identification of elements with atomic numbers 43, 61, and 85. The search for these elements among the rocks and minerals of the world was intense. Searching was not altogether random. The element 43 in particular, because of its reserved location in the framework of the periodic table, would have properties similar to those of manganese. Mendeleyev had tentatively named the element ekamanganese and predicted some of its chemical and physical properties. Scientists concentrated their search on ores and minerals known to contain those elements whose chemical behavior would be close to that of the missing element.

Several claims were made for the discovery of element atomic number 43 (which was later named technetium), the earliest of which was in 1887. All claims, however, were subsequently proved false until 1925, when Walter Noddack, Ida Tacke, and Otto Berg, based on unobserved X-ray spectral lines, reported a new element identified as atomic number 43. Each chemical element emits, under proper experimental treatment, X rays uniquely characteristic of that element. The emissions of known elements had been studied thoroughly and their wavelengths and relative intensities tabulated. Given, for example, an unknown substance, one could generate the X-ray spectrum of the sample, match it to the tabulated values, and identify its composition. Recognition of a previously unreported X-ray spectrum warranted the claim that a new element had been discovered. Following Noddack, Tacke, and Berg’s initial claim of discovery, in which they named their element masurium, they attempted to isolate a pure sample of the element from the mineral columbite, the source material of their newly defined X-ray spectrum. Because they were unable to do this and without a pure sample to support their claim, their discovery was rejected.

During the period when the search for the natural occurrence of atomic number 43 was taking place, another seemingly unrelated series of scientific investigations was under way. Natural radioactivity had been observed first at the beginning of the twentieth century. Certain naturally occurring substances sent out emissions spontaneously; that is, they were radioactive. Notable among these was uranium. The study of radioactivity, the identification of radioactive emissions (now recognized as corning from the nucleus of the emitting atom), and potential implications of this phenomenon for humanity were beginning to be recognized. Among the early pioneers in the study of radioactivity and the structure of that atom was Enrico Fermi, an Italian physicist. Fermi received the Nobel Prize in Physics in 1938 for his work on the production of artificial radioactive elements. Upon bombardment of a stable chemical element with nuclear particles, frequently new and often unstable isotopes are formed. Artificial radioisotopes are of enormous interest not only for the information they provide regarding the structure of matter but also for their practical applications in industry, medicine, and other areas.

Emilio Gino Segrè was the first of many graduate students to receive a Ph.D. degree in physics under Fermi’s guidance. He later received the 1959 Nobel Prize in Physics with colleague Owen Chamberlain for their discovery of the subatomic particle the antiproton. Segrè collaborated with Fermi on several studies pertaining to the interactions of particles with matter. These studies provided him with considerable insight into various nuclear processes. While serving as director of the physics laboratory at the University of Palermo, Italy, he received a sample of irradiated molybdenum sent to him by Ernest Orlando Lawrence. Lawrence had developed the cyclotron, a huge device weighing hundreds of tons and costing millions of dollars, which is capable of propelling nuclear particles to great energies. When these high-energy particles strike a target, they interact with the target material, producing artificial radioactive isotopes. Study of these isotopes and their decay contributed greatly to the understanding of matter and to the applications of radioisotopes for both peacetime and military use. The Lawrence Radiation Laboratory at the University of California at Berkeley was and remains today a leading center for the study of nuclear processes.

In December, 1939, Lawrence sent Segrè the molybdenum metal sample that had been bombarded by deuterium nuclei for several months in the cyclotron. Segrè and his research group undertook to study the effects of this bombardment and to isolate and identify the artificial radioisotopes produced. Separation of radioisotopes by chemical means—the only means available in 1937—was not an easy task. The transmuted products are many and are themselves undergoing radioactive decay forming further substances. Often, the quantity of each transmuted element is negligibly small, too small to be weighed by ordinary means. The presence of these products is generally ascertained indirectly through the measurement of their characteristic radio or X-ray emissions. Because these emissions are frequently very similar, prior chemical separations are mandatory if one is to say with certainty that a particular radionuclide is present. The slightest contamination of one component by another as complete chemical separation is seldom if ever achieved; it clouds the observed emissions.

It had been predicted from theoretical considerations that technetium could be one of the radioactive products formed from bombardment of molybdenum with deuterons. Segrè and his group were prepared to observe among the several predicted products (including isotopes of zirconium, niobium, and molybdenum) previously unreported emissions that might be attributed to technetium. Following lengthy chemical separations involving fusion, precipitation, filtration, and volatilization, an emission activity was observed that could not be assigned to any known element. This activity was attributed to atomic number 43, and Segrè first reported the results in a coauthored paper in the Journal of Chemical Physics in September, 1937. A series of papers by Segrè and his associates followed this initial notice and further characterized technetium by identifying several of its isotopes and studying its chemical and radiochemical properties. In 1947, Segrè reported formation of technetium from uranium fission rather than bombardment of molybdenum. The first significant quantity of the element, gram amounts, was prepared by others in 1952 from uranium fission products. Because of the availability of fissionable uranium from nuclear reactors, it is this source from which technetium is now prepared in commercially available quantities. The radiochemical data of Segrè’s group could not be disputed, and the name technetium (symbol Tc) was given to atomic number 43.

Scientists ask whether or not technetium occurs naturally. The half-life of the longest technetium isotope, Tc-97, is 2.6 × 10⁶ years. The term “half-life” refers to the time interval for one-half of any newly formed radioactive substance to decompose by emission of radioactive particles or rays. In the second half-life interval, half of the remaining amount decomposes. It is estimated that radioisotopes with half-lives of less than 1.5 × 10⁸ years would be virtually undetectable considering the time interval between the present and when the earth was formed. Because of natural radioactive fission of uranium from interaction with cosmic radiation, however, it is suggested that some, albeit a small amount of, Tc can be found naturally. It has been estimated that this amount is on the order of 10^–13 gram per gram of uranium ore and that this amount was sufficient to produce the X-ray emissions reported by Noddack, Tacke, and Berg in 1925.

Significance

With the identification of technetium and later astatine by Segrè and his colleagues, the missing gaps in the periodic table of the elements were filled. Although some questioned whether human-made elements should be recognized as true chemical forms, their complaints were quickly dispelled; credit for the discovery of technetium was given to Segrè and his research group. As with other radionuclides—both natural and artificial—studies of their properties have enhanced the understanding of matter and its composition and decomposition. Technetium, through its spectral emissions, has been identified on distant stars. Given existing knowledge concerning the half-lives of the various technetium isotopes, these data provide evidence regarding the time origin and composition of these stars.

Elemental technetium alone and in various alloys exhibits the property of superconductivity; that is, it passes an electric current with negligible resistance. As with all superconducting materials, this property is exhibited only at temperatures approaching absolute zero, yet as developments in this field advance, technetium may find a role in the manufacture of superconducting magnets.

The greatest use of technetium is in the field of medicine as a radiochemical tracer. In 1985, Thomas C. Pinkerton and his coauthors noted in the Journal of Chemical Education that “a wide variety of tissues can be visualized with ^99mTc radiopharmaceuticals, including the kidneys, bones, lungs, heart, liver, brain, and thyroid. Although other radionuclides are used in nuclear medicine, of the millions of diagnostic imaging procedures conducted each year, over 80 percent involve the use of ^99mTc.” The metastable technetium isotope ^99mTc emits a 0.143 million electronvolt gamma radiation with a half-life of 6.0 hours, transforming itself into the more stable ^99mTc isotope. Combining ^99mTc into compounds uniquely essential for various body organs and functions allows one (by monitoring the emitted gamma radiation) to determine the extent and duration necessary for incorporating these compounds into various bodily functions. If in monitoring this incorporation, one finds deviation from the expected normal body utilization of these materials, an abnormality is indicated and appropriate medical treatment can be started. Because of its short half-life, the ^99mTc-containing compound rapidly loses its radioactivity, causing little if any long-term damage to the patient.

Technetium, with its numerous chemical oxidation states, forms a variety of chemical compounds, in particular those incorporating organic molecules similar to or identical to those found in body organs. Technetium is taken up easily in these organs and suited for a specific organ whose normal function is suspect. Studies on other animal species and on various plant functions also incorporate technetium containing radionuclides.

Scientists have amassed considerable information regarding the hazards of technetium and its proper and careful handling. As is true of all substances, technetium-containing compounds pose a potential danger both to humankind and to the environment if they are carelessly used.

Bibliography

Barr, Robert Q. “Technetium.” In Van Nostrand’s Scientific Encyclopedia, edited by Glenn D. Considine. 9th ed. Vol. 2. New York: Van Nostrand Reinhold, 2002. Brief summary of technetium traces its history, isolation, chemistry, and applications to industry and medicine.

Boyd, G. E. “Technetium and Promethium.” Journal of Chemical Education 36 (January, 1959): 3-14. Detailed account of the discovery, chemistry, and uses of these two radioactive elements.

Deutsch, Edward, Karen Libon, and Silvia Jurisson. “Technetium Chemistry and Technetium Radiopharmaceuticals.” In Progress in Inorganic Chemistry, edited by Stephen J. Lippard. Vol. 30. New York: John Wiley & Sons, 1983. Review of technetium includes information on its chemical reactions, electrochemistry, and chromatographic separation techniques. Briefly discusses radiopharmaceuticals.

Holden, Norman E. “The Delayed Discovery of Nuclear Fission.” Chemistry International 12 (September/October, 1990): 177-185. A very personal account of the activities of those associated with the events leading to the discovery of nuclear fission.

Kotegov, K. V., O. N. Pavlov, and V. P. Shvedov. “Technetium.” In Advances in Inorganic Chemistry and Radiochemistry, edited by H. J. Emeléus and A. G. Sharpe. Vol. 11. New York: Academic Press, 1968. Overview of technetium traces its history, nuclear and chemical properties, separation procedures, and uses.

Krebs, Robert E. The History and Use of Our Earth’s Chemical Elements. 2d ed. Westport, Conn.: Greenwood Press, 2006. Introductory text explains the importance of an understanding of the chemical elements and examines individual elements within their groups. Presents information on the discovery, history, and uses of each element.

Paneth, F. A. “The Making of the Missing Chemical Elements.” Nature 159 (January, 1947): 8-10. A discussion on the search for atomic numbers 43, 61, 85, and 93. Urges the discoverers of artificial elements to name their discoveries.

Pinkerton, Thomas C., et al. “Bioinorganic Activity of Technetium Radiopharmaceuticals.” Journal of Chemical Education 62 (November, 1985): 965-973. Detailed discussion of technetium imaging for studying functions in the thyroid, brain, kidney, liver, bone, and heart.