Transuranium Element

In 1940, the first element with an atomic number higher than 92 was found, element number 93, now known as neptunium. This set off a search for even heavier elements. In the 15 years between 1940 and 1955 eight more were found, going up to atomic number 101 (mendelevium). Most of this work was done at the University of California laboratories in Berkeley, led by nuclear chemists Albert Ghiorso and Glenn Seaborg. Since 1955, the effort to find new transuranium elements has continued, although with rapidly diminishing returns. As of 1995 a total of 18 transuranium elements had been made, ranging up to atomic number 110. While the first nine transuranium elements were discovered within a 15 year period, discovering the last nine took almost 40 years. It was not that the experiments took that long; they had to await the development of more powerful cyclotrons and other ion-accelerating machines. This is because these transuranium elements do not exist on Earth; they have to be made artificially in the laboratory.

The transuranium story began when nuclear fission was discovered by Otto Hahn and Fritz Strassman in Germany in 1938. Chemists were soon investigating the hundreds of new radioactive isotopes that were formed in fission, which spews its nuclear products over half the periodic table. In 1940, E. M. McMillan and P. Abelson at the University of California in Berkeley found that one of those isotopes could not be explained as a product of nuclear fission. Instead, it appeared to have been formed by the radioactive transformation—rather than the fission—of uranium atoms, and that it had the atomic number 93.

When uranium was bombarded with neutrons, some uranium nuclei apparently had become radioactive and had increased their atomic number from 92 to 93 by emitting a (negative) beta particle. (It was already known that radioactive beta decay could increase the atomic number of an atom.) Because uranium had been named after Uranus, the seventh planet from the sun in our solar system, the discoverers named their "next" element neptunium, after the next (eighth) planet. When McMillan and other chemists at Berkeley, including G. Seaborg, E. Segrè, A. Wahl, and J. W. Kennedy, found that neptunium further decayed into the next higher element with atomic number 94, they named it plutonium, after the next (ninth) planet, Pluto. From there on, new transuranium elements were synthesized by using nuclear reactions in cyclotrons and other accelerators.

These experiments become more and more difficult as atomic numbers increase. For one thing, if you want to make the next higher transuranium element, you have to have some of the preceding one to use as a target, and the world's supply of that one may be only a few micrograms—a very tiny target indeed. It's worse than trying to hit a mosquito at 50 yards with a BB gun. While the probability of hitting one of these target nuclei with a "bullet" atom is incredibly small, the probability is even smaller that you will transform some of the nuclei you do hit into a particular higher atomic number nucleus, because once a "bullet" atom crashes into a target atom many different nuclear reactions can happen. To make matters even worse, the target element is likely to be very unstable, with a half-life of only a few minutes. So it's not only an incredibly tiny target, it's a rapidly disappearing one. The mosquitoes are vanishing before your eyes while you're trying to shoot them.

The heaviest transuranium elements have therefore been made literally one atom at a time. Claims of discovery of new transuranium elements have often been based on the production of only half a dozen atoms. It is no wonder that the three major groups of discoverers, Americans, Russians, and Germans, have had "professional disagreements" about who discovered which element first. When organizations such as IUPAC and the ACS get into the act, trying to choose a fair name that honors the true discoverers of each element, the disagreements can get rather heated.