Although the radioactive elements uranium and thorium were discovered early in the history of the elements — 1789 and 1828, respectively, years before the advent of the periodic table — radioactivity itself was unknown until 1896 when Henri-Antoine Becquerel (1852-1908) in Paris found that uranium could expose photographic plates, even when protected by black opaque paper. The renowned Marie Curie (1867-1934) promptly made a study of all elements (that were known at that time) and determined that only two were radioactive — uranium and thorium. She and her husband Pierre Curie (1859-1906) took up a study of the uranium ore from St. Joachimsthal, Bohemia (now Jáchymov, Czech Republic) and discovered radium and polonium in 1898. The next year André Debierne (1874-1949) in the Curie research group discovered actinium in the same ore, and the following year in North America radon was discovered in 1900 by Ernest Rutherford (1871-1937) and Frederick Soddy (1877-1956) in Montreal, Canada.* Independently protactinium was discovered in 1917 by Otto Hahn (1879-1968) and Lise Meitner (1878-1968) in Berlin and by Frederick Soddy and John Cranston (1891-1972) in Aberdeen, Scotland. All these elements seemed to fit in the periodic table, filling remaining gaps. By the 1920s only elements 43, 61, 85, and 87 remained unknown (although several spurious claims had been made).
Marie Curie Pierre Curie Ernest Rutherford Frederick Soddy
The major technique in tracing these elements was simply following their radioactivity in various chemical fractions as analytical procedures were being developed. However, a difficulty emerged — a glut of elements was appearing, each with a different half-life. For example, Rutherford observed that radium decays through a series of steps, giving a sequence Ra (radium) → Rn (radon) → Ra-A → Ra-B → Ra-C → Ra-E → Ra-F → Ra-G. Rutherford’s finding led to the discoveries by other investigators of a plethora of new elements in other decay schemes during the first decade of 1900. These elements included “ionium,” “brevium,” “actinouranium,” “radiothorium,” “niton,” “actinon,” “thorium-X,” “uranium-X,” and dozens more. The confusing feature of all these newly discovered elements was that in many instances some of them had very similar, and perhaps identical, chemical properties — even though they had different half-lives. There were simply too many elements to fit in the periodic table!
In 1913 Frederick Soddy solved the problem. He conceived of the idea of an “isotope.” Isotopes (from Greek “isos” meaning “same,” and “topos” meaning “place”) are “in the same place” in the periodic table, having the same chemical properties, and yet having different nuclear properties. The term isotope was coined during a dinner party of the Soddys in Glasgow, by a Dr. Margaret Todd, a literary scholar (see figure above). When the neutron was discovered by James Chadwick (1891-1974) in 1932, the source of the isotopes became clear — same atomic number, but different atomic mass. Soddy called this sequence of radioactive decay, such as the one observed by Rutherford, a “stately procession of chemical evolution,” wherein an atom could decay by losing an α-particle (helium nucleus) which would move the element down the periodic table two atomic number units, or a β-particle (an electron) which would move the element up one atomic number unit. Rutherford’s sequence now could be understood: it was merely a sequence of evolving elements of radium through a series of isotopes finally settling with “radium-G”, which was actually stable lead (although it was lead-206, comprising only 24% of naturally occurring lead, which is a mixture of 204, 206, 207, and 208, with an atomic weight, i.e., average crustal atomic mass, of 207.2).
One more naturally occurring radioactive element was discovered in 1939 by Marguerite Perey, who worked in the Curie Institute in Paris. She discovered francium, with the amazingly short half-life of 22 minutes, in uranium ores in 1939.
All the remaining radioactive elements were produced artificially. The first was technetium (at. no. 43), which was produced by the bombardment of molybdenum by deuterons in the Berkeley 37-inch cyclotron. The technetium itself was isolated by chemical means by Carlo Perrier and Emilio Segrè in 1937 in the Royal Institute of Experimental Physics in Palermo, Italy. The rare earth promethium (at. no. 61) was produced in an atomic pile in Oak Ridge, Tennessee by Jacob Marinsky, Lawrence Glendenin, and Charles Coryell in 1945. With these two elements (43 and 61), the periodic table was finally complete through uranium.
Next came the artificial synthesis of the transuranium elements. Each of these was generally discovered by a team of scientists, although one or two would be given the chief designation as the discoverer(s). This story began at the University of California at Berkeley. The key persons at Berkeley were Glenn T. Seaborg (1912-1999) and Albert Ghiorso (1915-2010). Ghiorso, who began his research at Berkeley as a technician for Seaborg, was featured in The Guinness World Book of Records (2003, page 173) as being the discoverer of the most elements, viz., americium through seaborgium for a total of twelve (Seaborg was responsible for “only” ten transuranium elements). Seaborg at the time was the only person for whom an element was named while he was still alive, thus breaking a long tradition of chemical nomenclature. (Since then, oganesson, at. no. 118, was named for Yuri Oganessian, the famous Russian scientist (1933- ).
The research of Niels Bohr (1885-1962) in Copenhagen, Denmark, clarified the periodic table for the heavier elements. Bohr’s model of the atom with s, p, d, and f shells allowed Seaborg to promote the introduction of the actinides, to be situated as a separate row beneath the rare earths.
Albert Ghiorso Glenn Seaborg
After the heyday of Berkeley’s research on the transuranium elements, Darmstadt, Germany (GSI Centre for Heavy Ion Research) and then Dubna, Russia (Joint Institute for Nuclear Research) became prominent in the synthesis of new heavy elements. Later participation of Japan (Riken, Wako, Japan) has also been important. Sometimes discoveries, or verifications, were made by joint ventures of the various institutes. In honor of these four research centers, elements have been named for them – berkelium (at. no. 97), dubnium (at. no. 105), darmstadtium (at. no. 110), and nihonium (at. no. 113).
Will there be more elements beyond 118? The transuranium elements form a trend of decreasing stability as one progresses to larger atomic numbers, and one might doubt the possibility. However, the proposal of an “Island of Stability” promises that perhaps elements with “magic numbers” of protons and neutrons may allow surprising stability of elements beyond oganesson. (The “magic numbers” for both protons and neutrons are 2, 8, 20, 28, 50, and 82 — but it is unclear what the next number in the sequence should be.) Speculations have been made that superheavy elements with half-lives of billions of years might exist. . . but we will have to wait and see!
* Some sources (e.g. Wikipedia, en.wikipedia.org/wiki/Radon) suggest that Robert Owens (1870-1940), a co-worker of Rutherford, should share the credit.
