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Ephemeral superheavy atoms coaxed into exotic molecules

Posted on behalf of Katharine Sanderson.

If you were ever to get excited about a chemical reaction, now might be the time.

An international team led by Christoph Düllmann at the Johannes Gutenberg University in Mainz, Germany, has managed to make a chemical compound containing the superheavy element seaborgium (Sg) — which has 106 protons in its nuclei — and six carbon monoxide groups.

The resulting molecule, reported on 18 September in Science, could be the start of a new chemical repertoire for the manmade superheavy elements, which do not exist in nature.

These elements are interesting not only to nuclear physicists — who use them to test how many protons they can pack into one nucleus before mutual electrostatic repulsion makes it explode — but also to chemists. The protons’ electrostatic pull on the electrons orbiting the nucleus is stronger in these elements than it is in lighter ones. This means that the electrons whiz around the nucleus at almost 80% the speed of light, a regime where Einstein’s special theory of relativity — which makes particles more massive the faster they get — begins to have a measurable effect. “It changes the whole electronic structure,” says Düllmann, making it different from those of elements that sit directly above the superheavy elements on the periodic table (see ‘Cracks in the periodic table‘).

Some chemists therefore expect superheavy elements to violate the general rule that elements in the same column should have similar electron structures and thus be chemically similar.

It is a brave chemist who attempts chemical reactions with superheavy elements. These cannot be studied with normal ‘wet chemistry’ methods and ordinary bunsen burners because they are made in very small numbers by smashing lighter atoms together, and tend to be extremely unstable, quickly ‘transmuting’ into other elements via radioactive decay. But it can, and has, been done, and researchers have identified fluorides, chlorides and oxides of these elements.

The difference this time is that the chemical reaction was done in a relatively cool environment, and a different kind of chemical bond was formed. Rather than a simple covalent bond, where the metal and the other element share electrons, Düllmann made a compound with a much more sophisticated sharing of electrons in the bond, called a coordination bond.

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Nuclei of the superheavy element seaborgium were created from a beam of neon ions (top right) and slowed down in a gas-filled chamber (RTC), where they reacted with carbon monoxide to produce a new kind of molecule.
Credit: P. Huey/Science

Düllmann’s team used the RIKEN Linear Accelerator (RILAC) in Japan to make seaborgium by firing a beam of neon ions (atomic number 10) at a foil of curium (96). This process yielded nuclei of seaborgium-265 — an isotope with a half-life of less than 20 seconds — at a rate of just one every few hours.

The beam also produced nuclei of molybdenum and tungsten, which are in the same column of the periodic table as seaborgium. The team separated the resulting seaborgium, molybdenum and tungsten from the neon using a magnetic field, and sent them into a gas-filled chamber to cool off and react with carbon monoxide. Molybdenum and tungsten are known to form carbonyls (Mo(CO)6 and W(CO)6).

Using a technique called gas chromatography, the team found that the seaborgium formed a compound that was volatile and tended to react with silica, the way its molybdenum- and tungsten-based siblings would. This indirect evidence was enough to convince Düllmann that he had made the first superheavy metal carbonyl (Sg(CO)6). “It was a fantastic feeling,” he says.

In this case the prediction — which the experiment confirmed — was that special relativity would make the molecule behave more like its lighter counterparts than might analogous compounds of different superheavy elements.

In an accompanying commentary, nuclear chemist Walter Loveland of Oregon State University in Corvallis writes that similar techniques could be applied to other superheavy elements from 104 to 109. In particular, the chemistry of element 109 (meitnerium) has never been studied before, he notes.

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