The past 75 years have been some of the richest ever for the discovery of new elements. Since C&EN began publication in 1923, 26 elements have been added to the periodic table. On average, that's about one new element every three years.

The era spans a major change in the kind of elements being discovered. Hafnium (discovered in 1923) and rhenium (in 1925) were the last elements with stable nuclei to be added to the periodic table. Beginning with technetium, discovered in 1937, the new elements are radioactive in all of their isotopes. All but one of them were "discovered" shortly after they were artificially produced through nuclear transmutation reactions, although some, such as neptunium and plutonium, were later found also to occur in nature.

The Periodic Table

With the discovery of hafnium, element 72, the then-new field of atomic physics was becoming crucial to the discovery of new elements. Although not a rare element, hafnium was nearly impossible to distinguish chemically from the much more common element zirconium. Dutch physicist Dirk Coster and Hungarian chemist George Karl von Hevesy used Neils Bohr's theory of how electrons fill shells and subshells within atoms to predict differences in the two elements' X-ray spectra. They then demonstrated that they could detect X-rays from both elements in a sample of a zirconium-rich ore. They named the element after Bohr's hometown of Copenhagen (Hafnia in Latin).

The last element to be discovered in nature was francium, element 87, whose longest lived isotope has a half-life of only 22 minutes. That isotope, produced during the natural decay of uranium-235, was chemically separated from uranium ore by Marguerite Perey at the Curie Laboratory in Paris in 1939.

By the mid-1930s, spurred by the discovery of the neutron in 1932 and artificial radioactivity in 1934, nuclear scientists- some trained as chemists, others as physicists-were attempting to bombard heavy elements like uranium with neutrons in the hope of synthesizing new elements. Italian physicist Enrico Fermi and colleagues tried to do so in 1934 and thought they had succeeded. But four years later, Otto Hahn and Fritz Strassmann demonstrated experimentally that Fermi had instead discovered a new nuclear reaction, later named fission.

Nuclear particles are more likely to" stick" to a target nucleus if they are moving at high speed, something that became technically feasible in 1929 when Ernest O. Lawrence invented a circular accelerator for charged particles called a cyclotron. Construction of the first of these accelerators on the Berkeley campus of the University of California in 1931 set the stage for the synthesis of new elements. The first, technetium-the name means artificial in Greek-was produced at Berkeley in 1937 by bombarding molybdenum with deuterium nuclei (deuterons) and was identified in Italy by Emilio Segrè and Carlo Perrier. That was quickly followed by astatine, produced and identified in 1940 from the bombardment of bismuth with helium nuclei.

Efforts to produce a transuranium element-one with a nucleus heavier than that of uranium, element 92-finally succeeded in 1940 when Edwin M. McMillan and Philip H. Abelson bombarded uranium with neutrons to produce neptunium, element 93. A follow-on series of experiments bombarded uranium with deuterons to produce a heavier isotope of neptunium, which decayed by beta-emission to element 94, plutonium, identified by Glenn T. Seaborg, Arthur C. Wahl, and Joseph Kennedy in 1941.

Over the next three decades, the Berkeley researchers (relocated to the University of Chicago during much of World War II) produced and identified nine more transuranium elements. And three other new elements were identified during the development of nuclear weapons. Of these, promethium (element 61), a fission product of uranium, was identified in 1945 as part of the wartime plutonium production project at Oak Ridge, Tenn. Einsteinium and fermium (elements 99 and 100, respectively) were discovered in the debris from the test of the first hydrogen bomb in 1952 by researchers at Los Alamos National Laboratory. The fermium isotope discovered after this blast had been created by 17 successive neutron captures in uranium, followed by sequential beta-decay.

Number of elements keeps growing

One of the very few pieces of original research ever published in C&EN was Seaborg's proposal in 1945 that the elements heavier than actinium had been misplaced in the periodic table, where they had been located as transition-series metals. On the basis of the chemical properties of neptunium, plutonium, and the two elements that follow them-americium and curium-Seaborg proposed a second lanthanide-like series, which he named the actinide series.

Since the mid-1970s, synthesis of ever-heavier new elements has depended on new generations of particle accelerators. Three facilities have been the major centers for this work-the Joint Institute for Nuclear Research in Dubna, Russia; the heavy ion research facility in Darmstadt, Germany, known as GSI; and Lawrence Berkeley National Laboratory. Elements produced in experiments at these facilities have half-lives that range from seconds to milliseconds and sometimes are produced in quantities of fewer than 100 atoms. What constitutes positive identification of a new element in such circumstances has become an increasingly important question. The International Union of Pure & Applied Chemistry (IUPAC), which has final say on the names of elements, has become, in effect, the arbitrator in a number of disputes over first discovery.

Last August, IUPAC gave its endorsement to a compromise slate of names for elements 101 to 109. Some of the proposed names had been embroiled in considerable controversy, and IUPAC sought to spread the credit for element discovery. Thus, element 105 was named dubnium to acknowledge the many contributions of the Russian lab. The Berkeley group got two of the names it wanted-rutherfordium for 104 and seaborgium for 106. And the GSI group, headed by Peter J. Armbruster, got credit for discovering elements 107, 108, and 109 with the names bohrium, hassium, and meitnerium, respectively. Between 1994 and 1996, the GSI group claimed discovery of three more elements-110, 111, and 112- none of which has been named yet.