It was possible to obtain a real chemical compound from helium.
In the periodic table there are two groups of elements that are described by the same, seemingly completely non-chemical concept – we are talking about noble metals, which include gold, platinum and some others, and about noble gases.
Left: structure of the Na2He compound, where gray balls are He atoms, pink balls are Na atoms. Right: Electrode electron density map, where a localized electron pair is clearly visible (2e). (Photo: Nature Chemistry (2017) doi:10.1038/nchem.2716.) Saturn is a gas giant – the name given to planets consisting primarily of gases such as hydrogen, helium, ammonia, methane, etc. (Photo: NASA, public domain .) ‹ › Open full size
With the “nobility” of metals, everything is more or less clear: they, roughly speaking, do not rust, unlike iron. The same goes for gases: helium, neon, argon, krypton, xenon and radon are so “noble” that they do not enter into almost any chemical reactions with any other elements. For quite a long time they were considered absolutely inert chemical elements, although from time to time someone encroached on their inertness.
The first breach in the impenetrable fortress of noble gases was made by the American chemist Neil Bartlett, who used platinum hexafluoride as a “chemical weapon” – with its help he synthesized the world’s first chemical compound of xenon. Agree, it turns out very symbolically when a compound of a noble metal enters into a chemical reaction with a noble gas. However, let’s put the lyrics aside – the most important result here was that elements that were considered inert for almost a century, it turns out, can enter into chemical reactions! This spurred the interest of researchers in studying a completely new field – the chemistry of noble gases.
After some time, the “fortresses” of krypton, argon and even radioactive radon fell under the onslaught of researchers, but the two elements still continued to hold their own. Despite all the efforts of chemists, until recently they had not been able to obtain compounds of helium and neon. Of course, under special conditions it was possible to synthesize some molecular “surrogates”, such as excimers, clathrates or molecular ions, but they are still not fully chemical compounds, and the search for the “key” to the remaining pair of the most noble gases continued. And it seems that such a key has been found to the most inaccessible chemical element – helium.
Artyom Oganov and his colleagues from Nankai University, State University of New York at Stony Brook, Livermore National Laboratory, Moscow Institute of Physics and Technology and a number of other research centers were able to predict the existence of two stable helium compounds, and then confirmed their theoretical calculations on practice.
First, using computer calculations, they analyzed the stability of a system consisting of sodium and helium atoms under various conditions. The conditions turned out to be very unusual: the researchers suggested that if such structures could exist, it would only be at very high pressures, up to 1000 gigapascals (this is approximately 10 million atm.).
If the energy of the resulting compound is less than the energy of the original non-interacting atoms, then the structure can exist, and if the energy increases when atoms combine, then such a structure will be unstable. In other words, when it is better for atoms of chemical elements to be together than apart, then a new substance is formed in which the atoms are connected to each other by chemical bonds. And if it is more advantageous for atoms to be separated, then no stable connections will be possible.
Calculations have shown that at pressures above 113 GPa, a stable compound can appear in which there are two sodium atoms per helium atom: Na2He. The most amazing thing is that such a compound was obtained in practice by placing the necessary substances in a diamond anvil and creating the required pressure. However, the question arises: why, in fact, chemists are sure that the result is a chemical compound, and not just a forced juxtaposition of atoms, as in other structures, for example, in clathrates. And here there is convincing evidence in favor of the new chemical.
As we know, sodium is a metal with all the properties inherent in metals, the main one of which is the ability to conduct current. The current in metals is caused by free electrons that can “travel” within the substance. What happened when helium was added to sodium and compressed to colossal pressures? If helium atoms simply entered the sodium structure without interacting with it, then the properties of the metal would not undergo significant changes, at least such a substance would retain its metallic properties.
But in our case, the picture turned out to be completely different. Na2He turned out to be an electride crystal, in which the place of negatively charged ions is taken by localized pairs of electrons and which therefore cannot conduct current. Thus, helium atoms not only make the resulting substance stable (the atoms “feel good” together), but also radically change its electronic structure, which can be considered the formation of a chemical compound.
This discovery, in addition to the significant deprivation of helium of its “chemical integrity,” gives us completely new ideas about what processes can occur inside planets that are gas giants (which include, for example, Jupiter and Saturn). After all, helium is the second most abundant element in the Universe, and the more we learn about the states in which it can exist, the better we will understand how the world outside our planet works.
Finally, we emphasize once again that in this study the result was first obtained, figuratively speaking, on paper, and only then was confirmed in practice – in full accordance with the aphorism of the great physicist Gustav Kirchhoff, who said that “there is nothing more practical than a good theory” .
The research results were published in Nature Chemistry.