Redefining a key chemical concept

If a piece of copper falls into a glass of water, nothing happens, but if a piece of sodium falls into the water, a violent chemical reaction occurs, producing enough heat to melt the sodium. The essential reason for this difference is that sodium is exceptionally low in electronegativity, and it "can't wait" to give up its electrons.

Electronegativity is arguably one of the most important characteristics of chemical elements. They largely determine what elements react with and how they react, what type of chemical bonds will form, and what properties the resulting compounds will have. However, much of this knowledge is only applicable to chemistry under standard conditions.

Although we already have a fairly deep understanding of the behavior of matter under atmospheric pressure, when you think about it, this is not a typical situation at all. Much of the material on Earth, as well as from other planets, is in an alarmingly high-pressure environment, such as the pressure at the center of the Earth at almost 4 million atmospheres. Under high pressure, strange phenomena that violate the rules of classical chemistry begin to appear one after another.

Recently, a team of scientists working with Russia completed an amazing "challenge." They revised the key chemical concept of electronegativity and determined the electronegativity characteristics of all elements under different pressure conditions. The revised concept of electronegativity provides a unified theoretical framework for understanding the many anomalies of high-pressure chemistry. The study has been published in the Proceedings of the National Academy of Sciences.

Malikenbu electropositivity

Electronegativity, along with the concept of chemical hardness, which is closely related to it, is often considered to be two basic chemical properties.

The level of electronegativity reflects the tendency of atoms to produce or capture electrons in chemical reactions. This feature makes more sense when comparing. For two arbitrary elements, the greater the difference in electronegativity, the more intense their atomic reactions. This makes the electronegating "champion" (the most electronegating element) fluorine and the "crane tail" (the least negative, or most electropositive) cesium the two most active elements. They all react eagerly, so they rarely appear in their pure form in nature.

In 1934, Robert Mulliken defined electronegativity for various atoms. The electroponderity of Malikenpo is calculated from the ionization potential of an atom and its electron affinity potential. The former measures the difficulty of "pulling" an electron out of an atom, while the latter reflects the extent to which an atom is "willing" to grab an electron from the surrounding vacuum. Half of the sum of these two values is electronegativity, and half of the difference between them is the chemical hardness of the element.

Under standard conditions, electronegativity and chemical hardness are very similar because the electron affinity potential tends to be very small. As a result, chemical hardness is often overlooked. But the problem is that once the pressure rises, things become different.

Abnormalities under high pressure

Scientists have found that many anomalies occur under sufficiently high pressures. For example, every substance becomes metal. Interestingly, at 2 million atmospheres, metallic sodium first becomes dielectric and then metallizes again at greater pressure. Inert gases at high pressure are no longer inert, they do form compounds.

Many elements become electron compounds, which means that they banish electrons into lattice voids, giving crystals special properties. Any two elements, even the seemingly boring sodium and chlorine in table salt (NaCl), form incredible compounds governed by some mysterious rule. Among these anomalous substances is record-breaking high-temperature superconductors.

The team realized that because the pressure affects the electron configuration of an atom, its electronegativity will change accordingly, which is actually quite natural. The previously proposed definition of electroponderivity in Malikenbu no longer applies at high voltages. They decided to set out to study the law by which electronegativity changes as stress increases.

Redefine electronegativity

The team managed to explain these strange phenomena by modifying basic chemical concepts of electronegativity and chemical hardness. They modified the definition and measured the electronegativity and chemical hardness of each element in the range of zero to 5 million atmospheres in the periodic table before 96.

Under high pressure, these two parameters diverged and had different physical significance. For solid materials, the chemical hardness is the band gap, which controls whether the material is metal, dielectric, or semiconductor. As for electronegativity, it means the chemical potential of electrons in atoms, that is, Fermi energy in the case of solids.

There are two special considerations for calculating this value under high pressure. First, pressure means that there is no vacuum, so the atomic ionization potential and affinity potential for vacuum electrons mentioned in the standard definition no longer apply. Thus, in the new definition, atoms are exchanged electrons with electronic gases, rather than vacuum. Second, they chose to replace the ionization potential and affinity potential with enthalpy values, which are critical to making meaningful predictions at pressure.

In determining the electronegativity of all elements at high voltages, the team faced the challenge of going beyond theoretical complexity. The researchers recalled the difficulty of one of the experiments. Malikenpo is a property of isolating atoms in a vacuum, but how do you place an atom under enormous pressure while still keeping it largely isolated from outside influences?

They found a trick to confine the atoms to a pressure chamber made up of helium atoms. Helium atoms are inert, and helium atoms are small, so the pressure is evenly distributed. The researchers then measured the energy, or enthalpy, of electrons detaching and joining atoms, and used this data to calculate electronegativity and chemical hardness.

In the end, their newly proposed electronegativity and chemical hardness scale results successfully explained the astonishing phenomena of non-classical chemistry that could not be explained before.

Just a few examples. In the case of increased pressure, the chemical hardness tends to decrease. This is explained by the narrowing of the band gap and pushing each element to eventually become a metal.

Electronegativity also has a downward trend under high pressure, which means that atoms become more "willing" to lose electrons. As atoms are compressed, there is less and less room for electrons. At some point, they have nowhere to go and are expelled into the lattice void. This produces electron compounds.

Even under extreme pressure conditions, fluorine remains the undisputed champion of electronegativity. As for the most electropositive atoms, cesium is surpassed by sodium. In some cases, magnesium also topped the list. When the pressure is high enough, this violates the periodic law. The enormous electronegativity of sodium and magnesium under pressure gives them incredible reactivity.

Long study

The study was conducted on and off, taking a total of nearly seven years. Not only does it involve a lot of deep thinking, but it also requires precise and complex calculations. But in the end the hard work paid off.

By modifying these two core concepts in chemistry, the team has successfully explained a series of confusing phenomena with a unified theoretical approach and generated new hypotheses that have implications for geology, planetology, and other sciences.

#创作团队:

Written by: Gaviota

Typography: Wenwen

#参考来源:

https://www.skoltech.ru/en/2022/03/56764/

#图片来源:

Cover image: Skoltech

First image: Skoltech