The inert element xenon has been forced into partnership with the humble water molecule
Xenon has more claims to fame than just being the only element beginning with the letter x. It is also part of a curmudgeonly group of elements, the noble gases, known for their unreactivity, and it is also at the centre of a geological mystery - there is a lot less xenon in the atmosphere than there should be.
A group of chemists and physicists have for decades been doing their best to debase xenon’s nobility by forcing it to mix with hoi polloi. Since Neil Bartlett made the first xenon compound — xenon hexafluoroplatinate — in 1962, around 80 compounds have been made, many of them with the aggressive fluorine atom. The latest step forward is to insert atoms of xenon into a water molecule — and in doing so, the scientists involved may have gone some of the way to solving the mystery of the missing element.
The molecule in question, HXeOXeH, contains two xenon atoms bound to a single oxygen atom. Leonid Khriachtchev, from the University of Helsinki, Finland, who led the research, says that this is the smallest known neutral molecule containing two xenons ever made. Chemists have inserted single xenon atoms into water molecules since 1999, using a reaction of water and a solid xenon matrix, driven by UV light. Khriachtchev noticed that this reaction produced a very reactive excited radical molecule with the formula HXeO. This can react with another xenon atom, and more hydrogen, to make the double-headed xenon molecule that Khriachtchev now reports in the Journal of the American Chemical Society1. Previously, he had used a similar technique to insert xenon atoms into acetylene, making HXeCCXeH.
What's goin' xenon?
It’s important to know that a noble gas radical like HXeO can go on to react with more noble gas atoms, says Khriachtchev. And he was “pleasantly surprised” to see that his xenon/water molecule was so stable, he says, although in this case stable still means that it falls apart if warmed above 70 K (-203 °C). But Khriachtev says he thinks that, theoretically, the molecule could exist at the same temperature that ice freezes.
Another way to make unstable molecules hang around for longer, apart from making them at very low temperature, is to put them under a lot of pressure. If the molecules can exist in these conditions, they might be able to help explain the “missing xenon problem”. “People don’t know where xenon is in our planet,” says Khriachtchev. One explanation is that xenon is there, but has been trapped in the crust as some kind of mineral.
Because water is abundant, a water-related xenon molecule could be evidence for this hypothesis says Khriachtchev. “I don’t claim that xenon is bonded to water [in Earth] but there’s a slight possibility and it should be investigated.”
In 2005, Chrystèle Sanloup at the University of Edinburgh, UK, suggested that squeezing quartz and xenon together at high pressure and temperature forces some of the silicon and xenon atoms to swap places, forming xenon minerals that could explain where the missing xenon went2. In her work, she didn't see any reaction with water, so is delighted that a reaction seems to occur. "It's great," she says. Knowing more about xenon reactivity will have an impact on Earth science, she adds, because xenon isotopes are often used to trace the formation of planets. "If xenon isn't that inert and gets trapped in the Earth, that method is wrong," Sanloup says. She is now considering returning to her experiments to see if, by squeezing a bit harder, she can prompt a xenon/water reaction in the rocks.
Khriachtchev wants to try and extend the chemistry of xenon by making binding repeated Xe–O units into a polymer. This kind of long chain is theoretically possible with two carbons in place of oxygen, he says.
As for the other noble gases, so far they are maintaining their reputation. Helium and neon, the two smallest and lightest ones, are the two elements that don’t form chemical bonds with anything. Only one example of an argon molecule has ever been made — again by Khriachtchev.
So far it has been impossible to carry out the water reaction with any noble element other than xenon, but that won’t stop Khriachtchev trying. His aim is to 'close' the periodic table: “We would like to make molecules with neon and helium, but we don’t know how to — yet,” he says. “Unfortunately even theoreticians can’t help us.”
Khriachtechev, L. et al. J. Am. Chem. Soc. 130, 6114–6118 (2008).
Sanloup, C. et al. Science 310, 1174–1177 (2005).