5 January 2007

Periodic Puzzling

By Rusty Rockets

Despite the periodic table's ubiquitous presence, how many people would have known what polonium (Po) was prior to the media circus surrounding the poisoning of Russian spy Alexander Litvinenko? The periodic table, which has symbolized chemistry ever since its controversial conception during the 1860s is largely thought of as a fixed reference work, but the table is yet to be completed, and some lucky scientists' careers involve running high-energy tests to fill in the gaps and perhaps catch a glimpse of the table's ultimate limits.

To UCLA chemist and historian Eric Scerri, author of the recently published The Periodic Table: Its Story and Its Significance, the periodic table symbolizes and encapsulates the whole field of chemistry. "It is completely unique in science. Chemistry is the only field with one simple chart that embodies the essence of the field. This wonderful tool serves to organize the whole of chemistry," he says. So while Dmitri Mendeleyev will always be known as the man who "invented" the periodic table, it's perhaps fitting that the table was actually the brainchild of six independent scientists. At least two of these gentlemen - English industrial chemist John Newlands and French mineralologist Alexander Beguyer de Chancourtois - arrived at the idea of conveniently arranging elements according to their atomic weight, so it would seem that a fetching arrangement of the elements was always on the cards.

And while attempts at formulating a feasible system for identifying recurring themes associated with known elements - hence the name "periodic" - often varied greatly among scientists, Scerri finds the idea that they did not influence Mendeleyev's final product unlikely. "I frankly don't believe it," he says. "Mendeleyev wasn't isolated in Siberia, which is the way he is sometimes portrayed. He spoke all the major European languages, was familiar with the literature and had traveled in Europe. He mentioned the precursors of the periodic table, but not the ones who actually devised systems. He surely must have known about them." So, perhaps the table should be considered an inevitable consequence of the evolving field of chemistry itself.

Naming rights aside, Scerri does give credit where he thinks it's due. "Mendeleyev's genius lay in his ability to sift intuitively through the mass of correct and incorrect knowledge of the elements that had accumulated to produce a system, an idea, that was both elegant and durable enough to withstand the chemical and physical discoveries that would follow," says Scerri. Scerri also adds that Mendeleyev managed to predict new elements, correct the atomic weights of some known elements, and correctly reverse the positions of the elements tellurium and iodine. Increasingly, however, relative atomic mass is being used to show the relationship between elements, as this takes into account all of an element's isotopes (the different forms that an element can take, such as uranium, which has isotopes 232 to 236 and 238) relative to their environment.

Initially, Mendeleyev published 30 versions of his periodic table, followed by roughly 700 published versions in the century that followed. Currently, the periodic table is comprised of 116 elements, with scientists continuing to push its boundaries. Scerri notes, for instance, the recent addition of element 118 (the "atomic number," or number of protons in an atom's nucleus) to the periodic table by a team of Russian and US researchers late last year.

Element 118 - positioned just below radon (86) on the periodic table - is now the heaviest of what are known as transuranium elements, or superheavy elements, those with atomic numbers above that of uranium (92). The identification of elements such as these would have been well beyond the capabilities and imaginations of Mendeleyev and his contemporaries, as a great deal of sophisticated equipment is needed to both generate and detect them.

In order to identify an element such as 118, researchers from the Joint Institute for Nuclear Research in Dubna, Russia, along with their colleagues from Lawrence Livermore National Laboratory in California, needed to bombard a target enriched in californium - number 98 on the table, so also a superheavy element - with a high-energy beam of calcium (20) ions. These experiments produce an exorbitant number of different species that survive for short periods before succumbing to decay due to spontaneous fission events (or elemental transmutation, where an atom splits into two or more smaller nuclei). After many thousands of hours of bombardment, the team could finally lay claim to identifying three correlated nuclear events that indicated the presence of three atoms of element 118. In this respect, both the formation and detection of this small number of atoms would not be possible without human intervention, and highlights the misconception that researchers are "discovering" new elements.

Ken Moody, a heavy-element group leader at the Livermore lab who worked on the 118 project, explains that the use of the word "discovery" in regard to these elements is somewhat of a misnomer. "These nuclides do not exist in nature, and as far as I know, could never have existed in nature," says Moody. "They are completely synthetic, and are created in the lab. However, when I was a student I had the opportunity to work with Glenn Seaborg for several years, and he considered the attempt to produce isotopes beyond the limits of the known nuclei very akin to a voyage of discovery, which has led to a common misuse of the word - poetic license, if you will."

However, some scientists believe that a number of transuranium elements can be formed in supernovas, which would be consistent with current knowledge about nuclear processes. "We know that nuclear explosives can make elements up to fermium (100)," says Moody. But due to certain constraints placed on their production, he explains, "eventually the uranium parent isotopes can no longer be produced because of the limits of nuclear stability [the neutron drip line]." It is these same effects that Moody believes also limits the production of elements beyond fermium in supernovas. "Even though production of these species is probably substantial, the transuranium elements have half-lives such that they mostly decay away before any supernova residue can find its way to us," says Moody. He explains that he expects there to be a limit to nuclear stability, and that as scientists continue to put more and more protons into the nucleus, the stability has to disappear.

Many chemists, however, believe that an "island of stability" does exist for superheavy nuclei; where such elements are comprised of a "magic number" of protons that makes them more resistant to decay. "The island of stability provides the means by which the heaviest known elements can avoid this [decay] process," explains Moody, while musing over what he and his colleagues will feel: "when we create the last nuclei, those perched on the edge of the void. Thankfully, this will probably not happen until after I retire." Until that time, Moody and his team "are not yet ready to give up," and next year they plan to bombard a plutonium target with iron projectiles to try to make element 120.

What will come of these experiments is anyone's guess, and applications for the elements that they produce are few and far between. "But these kind of experiments almost always pay off eventually in ways that the original experimenters could not have foretold," Moody noted.

Scerri points out the advances in theoretical physics and superconduction that followed British physicist J.J. Thompson's attempts to explain the order of the elements in the periodic table. "When chemists discovered high-temperature superconductors not too long ago, they wanted to raise the temperature even higher," says Scerri. "The way they did it was to simply look on the periodic table and to reason that if an element like lanthanum is a component of a superconductor, why not try actinium, which lies directly below it?" Who knows where these new super-heavy elements will lead future researchers and what intriguing new relationships will be revealed on the periodic table. "The periodic table is an icon for science, not just for chemistry, and it reflects deep truths about the elements," says Scerri.