A chemical glance at short-lived elements.
The chemical elements lawrencium, rutherfordium and hahnium hardly contribute to the stuff of everyday life. Synthesized one atom at a time by bombarding heavy nuclei with ions, these highly radioactive elements generally survive just a few seconds before decaying into other atomic isotopes.
Darleane C. Hoffman of the Lawrence Berkeley (Calif.) Laboratory and her co-workers, including collaborators in West Germany and Switzerland, have taken up the challenge of determining the chemical properties of these short-lived elements. Such studies enable researchers to see where the elements fit into the periodic table and whether they follow the trends in chemical behavior evident among lighter elements along the table's rows and columns. The researchers also look for evidence of new isotopes, in which the number of neutrons present in a nucleus differs from known varieties, and for traces of spontaneous fission, in which a newly synthesized nucleus immediately splits into two pieces.
Hoffman's team has been focusing on hahnium, or element 105. From its assigned position in the periodic table, hahnium ought to behave like its stable neighbor tantalum. However, recent experiments show that hahnium has a number of chemical properties resembling those of protactinium, a naturally occuring element situated beside uranium in the so-called actinide series. Protactinium is a dangerous, highly toxic material that requires precautions similar to those used when handling plutonium.
"Like plutonium, hahnium seems to have very nasty properties," Hoffman says. "There are many challenges in these studies of the heaviest elements." She described the results of her team's investigations at last week's American Chemical Society meeting in Washington, D.C.
The researchers synthesize hahnium by firing oxygen ions into a target consisting of berkelium-249. The resulting isotope, hahnium-262, has a half-life of 35 seconds, allowing only a tantalizingly brief interval in which to dissolve the isotope in water to study its chemistry in solution, or to react it with bromine to study its properties as a gas.
Like tantalum ions, hahnium ions in nitric acid stick to glass. However, whereas the organic solvent methyl isobutyl ketone readily extracts tantalum from solution, it fails to pull out hahnium. This result, along with hahnium's response to other solvents and the behavior of hahnium bromide in the gas phase, seems to put the element nearer to the protactinium camp than the tantalum group.
The findings support the notion that subtle quantum-mechanical and relativistic effects play an important role in establishing hahnium's electron arrangement, which determines the element's chemical properties. Such special effects are usually negligible in lighter elements.
"The nuclear and chemical properties of these [heavy] elements go hand in hand," Hoffman says. "They're hard to separate."
Hoffman would like to extend her chemical investigations to elements with a higher atomic number (a large number of protons) than hahnium, but the half-lives of known isotopes of elements 106, 107, 108 and 109 --sighted but not yet named -- are 0.8 second or less, often much less. That severely limits the time available for studying their chemistry.
However, researchers continue to search for heavy-element isotopes with significantly longer half-lives. The synthesis in 1986 of two relatively long-lived isotopes, lawrencium-261 and lawrencium-262, furnishes grounds for hope. These isotopes have half-lives 100 to 500 times longer than predicted by theory--sufficiently long to permit detailed studies of the chemical properties of lawrencium, the last member of the actinide series.
Lawrencium-262 also decays into nobelium-262, an isotope of element 102 containing more neutrons than any other known atomic nucleus.
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|Date:||Sep 8, 1990|
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