Transuranium elements and the periodic table
History of the transuranium elements
Cruising the transuranium highway
In chemistry, a transuranium (beyond uranium (U)) element, sometimes also called a transuranic element, is any of the chemical elements with atomic numbers higher than 92, which is the atomic number of uranium.
Ever since the eighteenth century when chemists began to recognize certain substances as chemical elements, uranium had been the element with the highest atomic weight; it had the heaviest atoms of all the elements that could be found on the Earth. The general assumption was that no heavier elements could exist on the planet Earth. The reasoning went like this: Heavy atoms are heavy because of their heavy nuclei, and heavy nuclei are unstable, or radioactive; they spontaneously transform themselves into other elements. Uranium and several even lighter elements—all those with atomic numbers higher than 83 (bismuth)—were already radioactive. Therefore, still heavier ones would probably be so unstable that they could not have lasted for the billions of years that Earth has existed, even if they were present when Earth was formed. In fact, uranium itself has a half-life that is just about equal to the age of Earth (4.5 billion years), so only one-half of all the uranium that was present when Earth was formed is still here.
If scientists could create atoms of elements beyond uranium, however, perhaps they would be stable enough to hang around long enough to study. A few years, or even hours, would do. However, in order to make an atom of an element with an atomic number higher than uranium which has 92 protons in its nucleus, scientists would have to add protons to its nucleus; one added proton would make an atom of element number 93, two added protons would make element 94, and so on. There was no way to add protons to nuclei, though, until the invention of the cyclotron in the early 1930s by American nuclear physicist Ernest O. Lawrence (1901–1958) at the University of California at Berkeley. The cyclotron could speed up protons or ions (charged atoms) of other elements to high energies and fire them at atoms of uranium (or any other element) like machine-gun bullets at a target. In the resulting nuclear smashup, maybe some protons from the bullet nuclei would stick in some of the hit target nuclei, thereby transforming them into nuclei of higher atomic numbers. That result is exactly what happened. Shooting light atoms at heavy atoms has turned out to be the main method for producing even heavier atoms far beyond uranium.
Such processes are called nuclear reactions. Using nuclear reactions in cyclotrons and other atom smashing machines, nuclear chemists and physicists over the years have learned a great deal about the atomic nucleus and the fundamental particles that make up the universe. Making new transuranium elements has been only a small part of it.
The road beyond uranium
Like any series of elements, the transuranium elements have similarities and differences in their chemical properties. Also like any other series of elements, they must fit into the periodic table in positions that match their atomic numbers and electronic structures. The transuranium elements are often treated as a family, not because their properties are closely related (although some of them are), but only because they represent the latest, post-1940 extension of the periodic table. Uniting them is their history of discovery and their radioactivity, more than their chemical properties.
Transuranium elements and the periodic table
Scientists can think of the atomic numbers of the transuranium elements as mileposts along a Transuranium Highway that begins at uranium (mile-post 92) and runs onward into transuranium country as far as milepost 116. As one begins the trip at 92, however, one realizes that the element is three mile-posts into another series of elements that began back at milepost 89: the actinides. Actinide Road runs from milepost 89 to 103, so it overlaps the middle of the 92 to 116 transuranium trip. (The road signs between 92 and 103 read both Actinide Road and Transuranium Highway.)
The actinides and all of the transuranium elements fit in periods 6 and 7 on the Periodic Table. The names that go along with the symbols of the elements from 93 to 109 are: Np = neptunium, Pu = plutonium, Am = americium, Cm = curium, Bk = berkelium, Cf = californium, Es = einsteinium, Fm = fermium, Md = mendelevium, No = nobelium, and Lr = lawrencium. Names for elements 104 to 109 are: Ru = rutherfordium, Db = dubnium, Sg = seaborgium, Bh = bohrium, Hs = hassium, and Mt = meitnerium. Elements 110 amd 111 are: Ds = darmstadtium and Rg = Roentgenium. Elements 111 to 116 have only been temporarily named while they wait to be independently confirmed. Their temporary names are: 112 = ununbium, 113 = ununtium, 114 = ununquadium, 115 = ununpentium, and 116 = ununhexium. (Other super-heavy elements, which have been predicted to exist [such as elements 117 and 118], have yet to be completely created in the laboratory, although research continues into the creation of these elements.)
The names of some of the transuranium elements and who discovered them have been the subjects of a raging battle among the world’s chemists. In one corner of the name-game ring is the International Union of Pure and Applied Chemistry (IUPAC), a more-or-less official organization that among other things makes the rules about how new chemicals should be named. In another corner is the American Chemical Society (ACS) and most of the American and German scientists who discovered transuranium elements. The names listed above are the ACS recommendations.
History of the transuranium elements
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, 19 transuranium elements had been made, ranging up to atomic number 111. Between 1996 and 2006, another five have been discovered but have yet to be confirmed. While the first nine transuranium elements were discovered within a 15-year period, discovering the last ten 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 reason 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 German scientists Otto Hahn (1879–1968)and Fritz Strassmann (1902–1980) 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, American scientists Edwin M. McMillan (1907–1991) and Philip H. Abelson (1913–2004) 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 Earth’s 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. Segreè, 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 (since then, Pluto has been demoted to a dwarf planet). 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 scientists to make the next higher transuranium element, they 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 is 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 one will transform some of the nuclei that is 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 is not only an incredibly tiny target, it is a rapidly disappearing one.
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.
Cruising the transuranium highway
Following is a brief sketch of each of the transuranium elements. Nuclear chemists using incredibly ingenious experiments have, in most cases, determined the chemical properties of these elements. They often work with one atom at a time, and with radioisotopes that last only a few minutes. The chemical properties of these elements will be omitted, however, because they are not available in sufficient quantities to be used in any practical chemical way; only their nuclear properties are important.
Neptunium (93) is named after the planet Neptune, the next planet after Uranus, for which uranium (92) was named. It was discovered in 1940 by McMillan and Abelson at the Radiation Laboratory of University of California, Berkeley (now called the Lawrence Radiation Laboratory), as a product of the radioactive decay of uranium after it was bombarded with neutrons. The neutrons produced uranium-239 from the ordinary uranium-238. The resulting uranium-239 has a half-life of 23.5 minutes, changing itself into to neptunium-239, which has a half-life of 2.35 days. Trace amounts of neptunium actually occur on Earth, because it is continually being formed in uranium ores by the small numbers of ever-present neutrons.
Plutonium (94) was first found in 1940 by Seaborg, McMillan, Kennedy, and Wahl at Berkeley as a secondary product of the radioactive decay of neutron-bombarded uranium. The most important isotope of plutonium is plutonium-239, which has a half-life of 24, 390 years. It is produced in large quantities from the neutron bombardment of uranium-238 while ordinary nuclear power reactors are operating. When the reactor fuel is reprocessed, the plutonium can be recovered. This fact is of critical strategic importance because plutonium-239 is the major ingredient in nuclear weapons.
Americium (95) was named after the Americas because europium, which is just above it in the periodic table, had been named after Europe. It was found by Seaborg, James, Morgan, and Ghiorso, in 1944, in neutron-irradiated plutonium during the Manhattan Project (the atomic bomb project) in Chicago, Illinois.
Curium (96) was named after French scientist Marie Curie (1867–1934), the discoverer of the elements radium and polonium and the world’s first nuclear chemist, and her husband, French physicist Pierre Curie (1859–1906). It was first identified by Seaborg, James, and Ghiorso in 1944 after bombarding plutonium-239 with helium nuclei in a cyclotron.
Berkelium (97) was named after Berkeley, California. Thompson, Ghiorso, and Seaborg discovered it in 1949, when they bombarded a few milligrams of americium-241 with helium ions. By 1962, the first visible quantity of berkelium had been produced. It weighed three billionths of a gram.
Einsteinium (99) was named after German– American physicist Albert Einstein (1879–1955). It was discovered by Ghiorso and his coworkers at Berkeley in the debris from the world’s first large thermonuclear (hydrogen bomb) explosion, in the Pacific Ocean in 1952. About a hundredth of a microgram of einsteinium was separated out of the bomb products.
Fermium (100) was named after Italian-American physicist Enrico Fermi (1901–1954). It was isolated in 1952 by Ghiorso, who was working with scientists from Berkeley, the Argonne National Laboratory, and the Oak Ridge National Laboratory, from the debris of a thermonuclear explosion in the Pacific Ocean. It was also produced by a group at the Nobel Institute in Stockholm by bombarding uranium with oxygen ions in a heavy ion accelerator, a kind of cyclotron.
Mendelevium (101) was named after Russian chemist Dmitri Mendeleev (1834–1907), originator of the periodic table. Ghiorso, Harvey, Choppin, Thompson, and Seaborg at Berkeley made it in 1955 by bombarding einsteinium-253 with helium ions. The discovery was based on the detection of only 17 atoms.
Nobelium (102) was named after Swedish inventor Alfred Nobel (1833–1896), the discoverer of dynamite and founder of the Nobel prizes. It was produced and positively identified in 1958 by Ghiorso, Sikkeland, Walton, and Seaborg at Berkeley, when they bombarded curium with carbon ions. It was also produced, but not clearly identified as element 102, by a group of American, British, and Swedish scientists in 1957 at the Nobel Institute of Physics in Stockholm. IUPAC hastily named the element for the Swedish workers. The Berkeley chemists eventually agreed to the Swedish name, but not to the Swedes’ credit for discovery. Ironically, in 1992 the International Union of Pure and Applied Chemistry (IUPAC) and of International Union of Pure and Applied Physics (IUPAP) credited the discovery of nobelium to a group of Russian scientists at the Joint Institute for Nuclear Research at Dubna, near Moscow.
Lawrencium (103) was named for American nuclear physicist Ernest O. Lawrence (1901–1958), inventor of the cyclotron. It was produced in 1961 by Ghiorso, Sikkeland, Larsh, and Latimer at Berkeley by bombarding californium with boron ions.
Many of the identities of the discoverers of elements 104 to 116 are tangled in an assortment of very difficult experiments performed by different groups of scientists. These groups include the American Lawrence Radiation Laboratory in Berkeley, the German Gesellschaft für Schwerionenforschung (Institute for Heavy-Ion Research) in Darmstadt, the Russian Joint Institute for Nuclear Research in Dubna, and the Swedish Nobel Institute of Physics in Stockholm. Other elements have not yet been independently discovered nor confirmed.
At this time, to name a transuranium element a researcher or team of researchers must be certified by IUPAC as the discoverers of that element, at which time they are free to name the compound. The elements 104 through 109 were subject to a naming controversy. The originally proposed names of these elements by IUPAC were, in order, dubnium, joliotium, rutherfordium, bohrium, hahnium, and meiterium. The names that appear on the current periodic table are, in order, rutherfordium (Rf), dubnium (Db), seaborgium (Sg), bohrium (Bh), hassium (Hs), and meitnerium (Mt).
A particular controversy among these elements involved element 106 that researchers at Berkeley were credited with discovering by IUPAC. Following historical convention, the Berkeley researchers were free to name the element. They chose to name it seaborgium, after Glenn T. Seaborg who contributed to the element’s discovery. IUPAC ignored the recommendations of the discoverers and suggested the name rutherfordium for element 106. A vote of the IUPAC Council in August 1995 resolved the issue, and now element 104 is called rutherfordium and element 106 is called seaborgium.
The end of the road?
The Transuranium Highway would appear to be coming to a dead end for two reasons. Chemists do not have large enough samples of the heaviest transuranium elements to use as targets in their cyclotrons, and the materials are so radioactive anyway that they only last for seconds or at most a few minutes.
Element 110 has been made by a slightly different trick—shooting medium-weight atoms at each other. The nuclei of these atoms can fuse together, and hopefully stick, to make a nucleus of a transuranium element. In spite of this gloomy picture, nuclear chemists are trying very hard to make much heavier, superheavy elements. There are theoretical reasons for believing that they would be more stable and would stick around much longer. The Transuranium Highway may be still under construction.
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Robert L. Wolke