Source of the Actinide Concept

ARQ Second Quarter 1997

My romance with the transuranium elements started 63 years ago, in 1934, soon after I became a chemistry graduate student at the University of California, Berkeley. These were the undiscovered elements with atomic numbers greater than 92 (the atomic number of uranium), the heaviest naturally occurring element.

We (the transuranium elements and I) were first introduced at the weekly chemistry seminar on nuclear science held in venerable Gilman Hall. Actually, I was introduced to what were thought to be the transuranium elements. I read articles by Enrico Fermi and coworkers about the induced radioactivities observed when elements such as uranium were bombarded with neutrons. Since some were published in their native Italian, they were a challenge to decipher.

These induced radioactivities were, of course, produced in trace (unweighable) quantities, so radiochemistry methods were needed. For guidance, researchers predicted the chemical properties using the periodic table as it was then known. The heaviest natural elements, thorium, protactinium, and uranium (atomic numbers  90, 91, and 92), were placed in that table just below the sixth-period “transition elements”—hafnium, tantalum, and tungsten (in these elements, the “5d” electron shell is being filled). Thus it was assumed that the 6d electron shell was being filled in these heaviest elements, and the chemical properties of the transuranium elements, the undiscovered elements 93, 94, 95, and 96, would be homologous with the 5d elements immediately above them in the periodic table, rhenium, osmium, iridium, and platinum. The limited chemical identification experiments of Fermi and coworkers seemed consistent with this view. The work of Otto Hahn, Lise Meitner, and Fritz Strassman in Berlin seemed to further confirm it. Little did we know then how we were being misled by accepting what was easiest to accept. I bought this interpretation “hook, line, and sinker.” In the fall of 1936, I described the work and interpretation of Otto Hahn and coworkers during a required graduate student talk to the chemistry faculty, staff, graduate students, and visiting scientists.

Then in January 1939, the bubble burst! At the physics journal club meeting, we heard something extraordinary. Niels Bohr, who had arrived in New York the previous week, brought news from Otto Hahn’s laboratory that the neutron- bombardment of uranium produced isotopes of light elements, like barium and lanthanum. The meaning was simple: the uranium had been split approximately in half, and all the radioactive “transuranium” isotopes studied by Hahn, Strassman, and Meitner during the previous four years were actually isotopes from the middle of the periodic table. This was exciting! After the seminar, I walked the Berkeley streets for hours, chagrined that I hadn’t recognized that the “transuranium elements” in which  I had been so interested were nothing of the kind. I felt stupid for failing to admit the possibility. Subsequent work showed that the radioactivities that had been ascribed to transuranium elements were actually due to fission products!

With poetic justice, the actual discovery of the first transuranium element resulted from experiments aimed at understanding the fission process. In 1940, Edwin M. McMillan and Philip H. Abelson showed that a radioactive product of the bombardment of uranium with neutrons was an isotope of element 93, with a mass number 239 (²³⁹93). The isotope ²³⁹93, a negative beta-particle emitter, should decay to the product ²³⁹94, but they were unable to observe this daughter product because of its long half-life.

McMillan then started looking for a shorter-lived isotope of element 94 through the deuteron bombardment of uranium. When McMillan was called to MIT for war work, I continued this quest with the help of my graduate student Arthur C. Wahl and another instructor in chemistry at Berkeley, Joseph W. Kennedy. We succeeded on the night of February 23–24, 1941, in chemically identifying (i.e., discovering) element 94 (the isotope ²³⁹94) in room 307, Gilman Hall (designated as a National Historic Landmark on the 25th anniversary of the discovery). Most importantly, we found that the chemical properties of element 94 weren’t like those predicted from the periodic table of that time (i.e., not like osmium), but were chemically similar to uranium. Joined by physicist Emilio Segrè, we soon identified ²³⁹94 and, most importantly, demonstrated that it was fissionable by slow neutrons.

“ This bold revision of the periodic table was a hard sell. When I showed it to some world-renowned inorganic chemists, I was advised not to publish it— such an act would ‘ruin my scientific reputation’. ”

Following McMillan’s suggestion for naming element 93 “neptunium” (after Neptune, the first planet beyond Uranus), with the chemical symbol Np, Wahl and I suggested “plutonium” (after Pluto, the next planet) for element 94. We first debated whether the name should be “plutium” or “plutonium,” the sound of which we liked better. Although the chemical symbol might have been “Pl,” we liked the sound of “Pu,” for the reason you might suspect, and therefore decided on “Pu.”

I had the pleasure of meeting for the first time Clyde Tombaugh, the discoverer of the planet Pluto, in Albuquerque, New Mexico, on June 9, 1991. At that time, he told me he had also considered naming his planet after the Greek god Cronus or Roman goddess Minerva (rather than after Pluto). In that case, I suppose we would have given element 94 the name “cronium” or “minervium,” and therefore, people throughout the world would never have heard the word “plutonium” which is so much in the news today.

The chemical properties of neptunium and plutonium were found to be similar to those of uranium and quite unlike those of rhenium and osmium, which, according to the existing periodic table, they should have resembled. Thus we concluded that a new series of 14 rare-earth-like elements, starting at uranium, would be the “uranide” (uranium-like) series, just as the 14 rare-earth elements were known as the “lanthanide” (lanthanum-like) series. Wrong again!

Soon after Pearl Harbor and the U.S. entry into World War II, I moved to the wartime Metallurgical Laboratory of the University of Chicago. Here, we solved many of the problems attendant with plutonium-239 production, and I turned my attention to the quest for the next two transuranium elements, 95 and 96. I was joined in the endeavor by my colleagues Albert Ghiorso, Ralph A. James, and Leon O. (Tom) Morgan. But when we predicted the chemical properties on the basis of the “uranide” concept, we failed to make any identification of our transmutation products. We weren’t successful until I suggested that we needed a bold revision of the periodic table in order to make correct predictions of the chemical properties of elements 95 and 96. I wrote a secret report in July 1944, suggesting that thorium, protactinium, and uranium be removed from the body of the periodic table and placed as the beginning of a “transition” series, analogous to the lanthanide (rare-earth) elements, in a separate row at the bottom.

Thus the 14 elements beginning with thorium (elements 90–103), would become the “actinide” elements (by analogy with the “lanthanide” elements). They would then show the necessary element-by-element analogy with the lanthanide elements (58–71). Thus element 95 would be chemically similar to the lanthanide element europium (63) and element 96 would be chemically similar to gadolinium (64). Using this concept, in 1944 and 1945, we synthesized and chemically identified elements 95 and 96, by analogy with their rare earth homologues, europium (63) and gadolinium (64). The new elements were subsequently named americium (95) and curium (96) by analogy with the naming of their homologues.

This bold revision of the periodic table was a hard sell. When I showed it to some world-renowned inorganic chemists, I was advised not to publish it—such an act would “ruin my scientific reputation.” However, I did publish it after the war, and it became a guide for the chemical identification of most of the subsequent members of the actinide series. The series was predicted to end at element 103, and the subsequent investigations confirmed this. At element 104 (now known as rutherfordium), we jumped back up to the body of the periodic table, and rutherfordium took its place under hafnium (element 72). (This spot had been occupied by thorium before I moved it to a separate row at the bottom of the periodic table). Then we proceed across the periodic table along now-known elements 105–112, to undiscovered elements 113–118; element 118 will be a noble gas.

This form of the periodic table is accepted throughout the world and is now ubiquitous in wall charts and chemistry books. I am, needless to say, proud that US chemists recommended that element 106, which is placed under tungsten (element 74), be called “seaborgium.” I am looking forward to the day when chemical investigators will refer to such compounds as seaborgous chloride, seaborgic nitrate, and perhaps, sodium seaborgate. Fortunately, this name, after initial rejection, is now being accepted by the Commission on Nomenclature on Inorganic Chemistry of the Union of Pure & Applied Chemistry (IUPAC). This, then, is a brief account of the origin of the actinide concept for the placement of the fourteen elements beyond actinium (atomic numbers 90–103) in the periodic table.