Argon: Not So Noble After All

The element argon has always been a loner. It's one of the inert gases that normally exist as single atoms. But in the 23 August issue of Nature, chemists report that they persuaded argon to mingle a little and form a compound with other elements.

Argon--along with helium, neon, xenon, radon, and krypton--belongs to the so-called "noble" gases. Also called inert gases, they have complete outer electron shells and were believed not to react with other elements or compounds. Nobility didn't last forever, however. In 1962, chemists prepared a compound that contained xenon, and compounds containing radon and krypton soon followed. Now argon joins the list, although neon and helium have yet to sully their solitude.

Inducing argon to react wasn't easy, but theoretical chemists predicted that it would be possible. The team, led by Marrku Räsänen of the University of Helsinki in Finland, had to devise a way to bring these recalcitrant molecules together. The trick was to trap the argon atoms between two other atoms that longed for each other, in this case, hydrogen and fluoride.

To begin, the team slowed everything down by cooling argon atoms to 7.5 degrees above absolute zero. Then they added hydrogen fluoride molecules and separated the hydrogen atoms from the fluorine atoms with ultraviolet light. As the team heated the argon film to 19 kelvin, the hydrogen atoms began to stir. "We see clearly that hydrogen atoms start looking for something to react with," says Räsänen. But its intended partner, the fluorine atom, is almost always hidden behind an argon atom, so the hydrogen has to form a linear molecule with argon in between: HArF.

The team identified these new molecules by observing their infrared spectrum. The proof was the absence of frequencies that had been absorbed by vibrations in the bonds between the three atoms. Not that they had long to look: The molecule is very unstable--it immediately gives up its argon in favor of bonding with nitrogen or oxygen.

The experiment is an "excellent achievement," says chemist Gernot Frenking of the University of Marburg in Germany, one of the theorists who made calculations predicting the existence of argon compounds. But it is only halfway to creating a compound that "you can put in a flask at room temperature" and experiment with, Frenking says. "I still believe that this might be possible, but it will surely be difficult to make it," he says, adding that the technique may be able to create compounds of helium and neon.

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