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Einstein's invisible hand: Is relativity making metal act like a noble gas?

3:33pm, April 9, 2008
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Superheavy element 114 should be a metal. Controversial data from an experiment in Dubna, Russia, suggest instead that effects from Einstein's theory of relativity might make the element's chemistry closer to that of a noble gas, like radon. If the results are confirmed, it would be the most significant departure yet from the predictable patterns in the periodic table of the elements.

Uranium (element 92, for the 92 protons in its nucleus) is the element with the highest atomic number commonly found in nature. In the lab, scientists have created additional elements up to 118 (with the exception of 117).

An element's characteristic chemical reactions depend on the arrangements of its outermost electrons, and elements with the same outer electron arrangement share a column in the periodic table. Artificial elements such as 114, which was first made in the 1990s, also in Dubna, should be no exception. "Theory says that 114 ... should have properties similar to those of lead," which lies directly above it in the periodic table, says theoretical chemist Valeria Pershina of GSI, a heavy-element research center in Darmstadt, Germany.

The elements however are not identical, Pershina explains. In particular, nuclei with more protons attract electrons more strongly. Those electrons orbit faster, and according to Einstein's special theory of relativity, time for them stretches out. As a result, some of the electrons' orbits are tighter than in lighter elements, affecting that element's chemistry.

But such anomalies, which have been observed in heavy elements such as 105 and are even visible in gold, should not be so large as to threaten the element's standing in the periodic table, Pershina says.

In the current experiment, chemist Heinz Gäggeler of the Paul Scherrer Institute in Villigen, Switzerland, and his collaborators produced nuclei of element 114 with a particle accelerator at Dubna's Joint Institute for Nuclear Research. The accelerator shoots a beam of calcium nuclei onto a thin foil coated with plutonium, Gäggeler says. Some of the calcium nuclei fuse with plutonium nuclei, producing a handful of 114s per month. The nuclei zip into a container filled with argon gas, where they capture electrons and become neutral 114 atoms.

To test the element's chemistry, the researchers continually pump the argon through a tube coated inside with gold. The tube has a temperature gradient, going from 30° Celsius where the argon enters to –185°C at the other end.

Atoms of a metal such as lead would readily bind to the gold, so they would not go very far down the tube. A noble gas such as radon, on the other hand, goes happily alone and would only stick to the colder part of the tube, like your fingertip sticks to the inside of a freezer. Wherever a 114 atom lands, its nucleus will decay within seconds, releasing alpha radiation. The researchers can then detect where along the tube the atom stuck.

So far, the experiment has counted a handful of decays at the cold end but none at the warm end. This element "seems not to behave like lead but much more like a noble gas," Gäggeler says. If the results hold up, he adds, "it would be the first time that an element ... does not behave as you would naïvely expect on the basis of the rules governing the periodic table."

Pershina, however, is skeptical. Only for elements with atomic numbers in the 160s or 170s—far beyond current capabilities to produce—should relativity begin to subvert the periodic table, she says. But Gäggeler says he believes that with more data, his team will win skeptics over.

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