The Case Of The Missing Xenon

Canadian scientists have reported the first synthesis and definitive spectroscopic characterization of xenon dioxide, XeO₂, the so-called missing xenon oxide. The accomplishment adds credence to a proposed theory explaining what happened to most of the xenon thought to have been present when the planet formed but which has since mysteriously disappeared.

By some estimates, 90% of Earth’s primordial atmospheric xenon and more than 99% of the planet’s mantle xenon have gone astray, whereas neon, argon, and krypton are still around. Because xenon likely didn’t end up in Earth’s core, the prevailing theory holds that xenon—the most reactive of the noble gases—is sequestered in Earth’s crust. According to the theory, xenon displaces silicon in quartz (SiO₂) under high-temperature and high-pressure conditions and is trapped as XeO₂. But until now, the only known xenon oxides were XeO₃ and XeO₄.

David S. Brock and Gary J. Schrobilgen of McMaster University in Hamilton, Ontario, have now prepared XeO₂ by hydrolysis of XeF₄ at 0 °C (J. Am. Chem. Soc., DOI: 10.1021/ja110618g). The team analyzed the yellow-orange solid by Raman spectroscopy, using ¹⁸O labeling to prove that they indeed had XeO₂ and not one of the other oxides or a xenon oxyfluoride such as XeOF₂. The researchers determined that XeO₂ is not monomeric but instead has an extended polymeric network structure in which square-planar xenon atoms are each coordinated to four oxygen atoms.

Chrystèle Sanloup of Pierre & Marie Curie University, in Paris, who proposed and showed experimentally that xenon might be inserting into SiO₂, says that “these new findings have profound implications for the missing-xenon problem.” One of the important issues now solved is knowing the proper coordination number of xenon atoms in XeO₂, Sanloup notes.

In addition, geochemists use noble-gas abundances and isotopic ratios to assess atmospheric and deep-Earth geological processes, she says, assuming noble gases are inert under all conditions. The present findings “throw the last rock” on that basic assumption, Sanloup says, suggesting that scientists may need to revise the way they use noble gases to assess such processes.