Helium Not Such A Noble Gas After All, As Scientists Form Stable Compound

A model of the first helium compound. The purple balls are sodium atoms, the green cubes are helium and the red regions represent voids containing electron pairs pushed aside by the helium. Artem R Oganov

One of the first things everyone is taught in chemistry class has turned out to have an exception, the creation of helium-sodium compounds proves helium isn't truly noble. Noble gasses got their name because they were thought not to mix with the other elements, forming no compounds. Nevertheless, we have known since the 1960s that the heavier noble gasses will, under sufficient pressure, form stable compounds, but helium has always remained aloof. Until now.

Helium is the second most common element in the universe. It makes up a large proportion of stars and gas giants, making its behavior important. That behavior has always seemed simple. Metastable compounds of helium have been made, along with what are known as van der Waals compounds, but neither are considered true stable compounds, either because they don't last, or the helium atoms barely affect the electrons of the other atoms.

Yet a team led by Professor Artem Oganov of the Skolkovo Institute of Science and Technology, Moscow, conducted computer modeling that suggested at least two helium compounds are possible. Na2He and Na2HeO. A team from China, Russia, and the United States came together to see if they could make the first of these, using a diamond anvil to apply more than a million times atmospheric pressure.

They announced their success in the journal Nature Chemistry, showing that not only can helium and sodium atoms be forced to combine, but above 113 billion Pascals of pressure the combination is stable, something the authors expect will still be the case at pressures up to 10 times greater.

"The compound that we discovered is very peculiar: helium atoms do not actually form any chemical bonds, yet their presence fundamentally changes chemical interactions between sodium atoms, forces electrons to localize inside cubic voids of the structure and makes this material insulating," said first author Xiao Dong of Nankai University in a statement

Sodium on its own has what is called an “electron gas” with outer electrons freed from their atoms to move independently. The presence of the helium atoms pushes electrons away, creating an ionic crystal resembling salts.

Phenomenal forces exist towards the centers of giant planets. With abundant helium and significant amounts of sodium, it is possible that helium-neon compounds are formed, altering the chemistry deep within the planets in ways we do not yet understand.

The team tried modeling other possible combinations of helium and light elements, but only those that involved sodium appeared stable. They have yet to make Na2HeO, but they expect that not only can it exist, but it will be stable at pressures somewhat lower than that of Na2He.

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