HArF! Argon’s not so noble after all

Noble gases—radon, xenon, krypton, argon, neon, and helium—are snobs. Their atoms typically shun liaisons with other elements because they already have all the electrons they need, but none to share. Only when chemists engage in forced matchmaking do some of these gases react with other elements to form stable, neutral compounds.

Researchers at the University of Helsinki in Finland report in the Aug. 24 Nature that the formerly aloof argon has been coerced into the chemical equivalent of a shotgun wedding.

The scientists made the new compound, argon fluorohydride (HArF), by shining a strong ultraviolet light on frozen argon that contained a small amount of hydrogen fluoride. The light split some of the hydrogen fluoride molecules into hydrogen and fluorine atoms, which then combined with argon to form the new compound, says Markku Räsänen, an author of the report. The resulting mixture absorbed wavelengths of infrared light that theorists had predicted would be absorbed by hydrogen-argon and fluorine-argon bonds, thereby confirming the presence of the new molecule.

The HArF molecules are marginally stable and remain intact only when isolated within the matrix of frozen argon, admits Räsänen. If they warm above 27 kelvins or if they touch one another, they readily break apart.

“This is a remarkable achievement and yet is only a half step toward truly synthesizing an argon compound,” says Gernot Frenking, a chemist at Philipps-Universitåt Marburg in Germany.

The true success, he contends, is making a molecule that can survive at room temperatures and be handled as other chemical compounds are. Although theoretically possible, such molecules may be extremely difficult to produce.

Scientists have created stable neutral compounds that include xenon and krypton. They’ve also made molecules that contain radon atoms, but these compounds are short-lived because of radon’s 3.84-day half-life.

Now, the only remaining loners are neon and helium. Their atoms are smaller than their noble brethren’s and therefore hold onto their outer-shell electrons more tightly. Frenking expects that the experimental techniques used to make HArF could be applied to manufacture stable molecules that incorporate the two noble holdouts, but finding the right chemical partners may be difficult.

Even though fluorine is highly reactive, Frenking says the bond between fluorine and neon probably wouldn’t be strong enough to hold the two atoms together. Instead, chemists may have to turn to chlorine. Although less reactive than fluorine, chlorine won’t be as strongly repelled by neon because of the configuration of the atoms’ outer-shell electrons.

Any neon or helium compounds that chemists produce are unlikely to be stable at room temperature. Frenking says that these molecules—like HArF—would have to spend their lives isolated within a frozen matrix, maintaining an eternal cold shoulder.

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