New electronegativity scale revolutionizes high-pressure chemistry

(ORDO NEWS) — The Skoltech professor and his Chinese colleagues have revised the key concept of chemistry – electronegativity – and determined this value for all elements at various pressures. Numerous anomalies in high-pressure chemistry have been theoretically substantiated within the framework of the updated concept of electronegativity.

Electronegativity and the related concept of chemical rigidity are fundamental properties of elements that largely determine how and in what reactions atoms enter. “If you throw a piece of copper wire into a glass of water, nothing interesting will happen.

If, however, a piece of sodium is thrown into water, then a violent reaction will immediately begin, as a result of which so much heat will be released that the sodium will melt. The thing is that sodium has a very low electronegativity: it energetically donates electrons to other atoms, ”comments Skoltech professor Artem Oganov, co-author of the study.

Electronegativity – perhaps the most important property of an atom for chemistry – its readiness to donate (if it is low) or take away (high) electrons. This value makes sense in comparison: the more it differs between two elements, the more violently their atoms react.

Therefore, the most reactive substances are fluorine and cesium, the champions with the highest and lowest electronegativity, respectively. Their reactivity is so great that neither one nor the other is found in nature in its pure form.

The electronegativities of atoms give a pretty good idea not only of what is reacting with what, but of the type of chemical bond and the properties of the resulting compounds. But all this is under normal conditions.

“We know a lot about the behavior of matter at atmospheric pressure, but this is, in fact, an atypical situation,” says Oganov. — Most of the matter of the Earth and other planets is under tremendous pressure. At the center of the Earth, for example, it is almost four million times higher than atmospheric pressure.”

When the behavior of matter under such pressures was learned to be reproduced in laboratories and simulated on a computer, including the USPEX method of predicting crystal structures invented by Oganov, scientists began to discover exotic phenomena one after another that contradict classical ideas. In particular, under sufficiently high pressure:

All substances become metals. It is curious that sodium metal, when compressed to two million atmospheres, first turns into a dielectric, and then becomes a metal again with even stronger compression.

– Inert gases cease to be inert and form compounds. Even helium!

– Potassium and some other elements give rise to strange, non-periodic structures in which some of the atoms form a framework, and the rest fill the cavities and form chains in them. In this case, the periodicity of the framework and the chain does not coincide, that is, it is impossible to distinguish a repeating elementary cell from such a structure.

– Many substances become electrides, that is, they expel electrons into the voids of the lattice, which gives the crystal bizarre properties.

– Any pair of elements, including the banal sodium-chlorine system (table salt), forms strange compounds, such as Na 3 Cl and NaCl7, according to unknown rules. Among such anomalous substances, by the way, there are record-breaking high-temperature superconductors.

– Unusually high valences occur. Cesium, for example, becomes pentavalent, and copper becomes tetravalent.

– Elements that do not interact at atmospheric pressure begin to react: copper – with boron, magnesium – with iron, and so on.

Oganov and colleagues managed to explain these unusual phenomena by revising the fundamental concepts of chemistry – electronegativity and chemical rigidity. Scientists have noticed that the definition of electronegativity, introduced in 1934 by Robert Mulliken, is fundamentally applicable only at zero pressure.

By modifying this definition, they calculated electronegativity (as well as chemical hardness) at pressures from zero to five million atmospheres for all elements of the periodic table up to the 96th.

“These two parameters largely determine the chemical properties of atoms, and we decided to consider how they change with increasing pressure. The fact is that when an atom is compressed, the configuration of its electrons changes. And, of course, this is reflected in its electronegativity,” says Oganov.

The calculation of electronegativity according to Mulliken is based on the ionization energy of an atom (how difficult it is to tear off an electron from it) and electron affinity energy (how willingly an atom attaches an electron from a vacuum).

The half-sum of these quantities gives electronegativity, and the half-difference gives chemical rigidity, and under normal conditions these characteristics are close, because the electron affinity of most atoms is low. As a result, chemists usually do not consider chemical rigidity. But under high pressure, everything changes.

“At high pressures, these two parameters behave differently and have different physical meanings: for a solid, chemical hardness is the band gap, and it determines whether this substance will be a metal, a non-metal, or a semiconductor,” says Oganov. — Electronegativity has the meaning of the chemical potential of an electron in an atom (for a solid body, it is equal to the Fermi energy).

Its calculation under pressure must take into account two circumstances. First, vacuum is impossible under pressure, which means that the standard definition of ionization potential and electron affinity is not applicable.

Therefore, in our formula, instead of vacuum, we have an electron gas. Secondly, we replace the ionization energy and affinity in the formula with enthalpy, otherwise the predictions of the stability of elements under pressure will be false.”

When calculating electronegativity under high pressure, scientists encountered not only theoretical difficulties. “Mullekin electronegativity is a characteristic of an atom floating in the void, but if it is under enormous pressure, then, by definition, other atoms are pressing on it,” Oganov explains.

Without thinking twice, we placed the atoms in a large cell of helium atoms – this is the most inert thing we have. In addition, helium has small atoms, so the pressure is distributed evenly.”

Under the pressure of helium, the researchers calculated for each atom the energy – more precisely, the enthalpy – of the detachment and attachment of an electron, and from it they calculated the electronegativity and chemical rigidity.

“The work went on intermittently and took a total of almost seven years,” Oganov recalls. “We started it when the first author, Xiao Dong, was a graduate student in my lab. And they finished when he had already become a professor.

Here a huge amount of not only mental work was done, but also heavy calculations, but it was worth it. It turned out that the new scale of these quantities successfully explains the unusual phenomena of non-classical chemistry.

Since electron gas now serves as a conditional reservoir of electrons, it is logical that an atom with a negative electronegativity index will give electrons to the gas, with a positive one – take away, and with zero – be in equilibrium with the gas.

So for most metals, the electronegativity turns out to be close to zero, and this is in perfect agreement with the fact that their properties are usually described through the electron gas model.

The chemical stiffness of the elements falls under pressure – the band gap decreases, so sooner or later any element becomes a metal.

As the pressure increases, the electronegativity also decreases, the atoms donate electrons more easily. The atomic core is compressed, and there is less and less room for electrons. This is how electrides appear: in them, the electrons had nowhere to go and they were forced to huddle in the voids of the lattice.

In calcium, barium, strontium, potassium, sodium, under pressure, the chemical rigidity reaches very low values, which explains the ability to disproportionate into different types of atoms and the formation of strange structures consisting of a framework and chains, which together form a non-periodic crystal structure.

Under high pressure, fluorine remains the electronegativity champion. But the most electropositive atom is not cesium, but sodium.

“And at even more extreme pressures, magnesium joins it, which in a sense violates the periodic law, because magnesium is an element of another group in the periodic table,” Oganov comments on the results. This low electronegativity of sodium and magnesium under pressure makes them incredibly reactive.

In nickel, palladium and platinum, the two upper electron shells are redistributed in such a way that a completely filled d-electron shell appears. Since filled shells are particularly stable, these elements become less active and cease to form compounds with some elements with which they react under normal pressure.

This effect has even greater consequences for neighboring elements: atoms that lack one or two electrons (cobalt, iron, rhodium, ruthenium, osmium, iridium) acquire an unusually high electronegativity comparable to iodine and tellurium. And elements that have one or two “extra” electrons (copper, silver, zinc, cadmium) acquire very low electronegativity.

Between magnesium and iron under pressure, the difference in electronegativity increases as much as four times. Similar is the case with copper and boron. Hence the fantastic combinations of these elements. “We did a lot of tests,” says Oganov.

And yes, copper really easily reacts with boron and other elements. And cobalt and rhodium easily take away electrons from many metals. We think that all this can be very important for geochemistry, changing the geochemical behavior and fate of many elements.”

“Another observation: as the chemical rigidity decreases, the degree of localization of electrons on bonds decreases, and so-called multicenter bonds are formed. This, in particular, is associated with the formation of exotic compounds such as NaCl 7 ”, says the first author of the work, professor at Nankai University (China) Xiao Dong.

“And the last thing: although the atom gives up each subsequent electron more reluctantly than the previous one, the decrease in electronegativity and chemical rigidity under pressure leads to the fact that this effect is weakened, and this is why the pentavalent form of cesium, tetravalent copper, etc. become possible – all this also follows from updated electronegativity scale,” concludes the researcher.

Thus, the revision of the key concepts of chemistry not only makes it possible to explain, within the framework of a single concept, a lot of strange phenomena under high pressure, but also generates new hypotheses in the field of geology, planetary science and other sciences.

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