Chemists have found that some atoms are bound together in surprising ways, completely deviating from the definition of traditional chemical bonds.
Open any chemistry textbook and you'll find an introduction to chemical bonds. This is not surprising. Chemistry is a discipline that studies chemical reactions, and the essence of chemical reactions is the breaking and formation of chemical bonds. Therefore, chemical bonds can be described as the "foundation of life" of chemistry as an independent discipline.
Chemical bonds are the "bonds" formed by the transfer or sharing of the outermost electrons of atoms; just as a couple forms a family through marriage, through which they are bound to each other. Surprisingly, although the concept of chemical bonds has been proposed for a century and a half, we are far from fully understood. For example, traditional chemical bonds only have several types of ionic bonds, covalent bonds, metal bonds, etc.; but recent discoveries have shown that there are more types of chemical bonds than we think; and some familiar chemical bonds may not be the same as we originally thought; and even some bonds, completely contrary to the orthodox concept of chemical bonds, do not involve electrons at all.
These new chemical bonds will not only allow us to better understand existing substances, but also help us unlock the potential of chemical elements, providing a whole new world of materials for solar cells, drugs, data storage, and many more applications.
A brief history of chemical bonds
As early as the early 1860s, chemists had begun to write the names of chemical substances in the form of symbols connecting elements with short horizontal lines. For example, H-H represents the hydrogen molecule H, which consists of two hydrogen atoms. By 1866, the British chemist Edward Frankland had invented the term "bond" to describe the relationship represented by these dashes.
At that time, there was still controversy about whether atoms really existed. No one envisioned the picture of an atom that we are familiar with today, of "a swarm of electrons outside the nucleus moving around a single nucleus." The concept of covalent bonds, the idea of two atoms bonding by sharing electrons, was first proposed in the early 20th century. It wasn't until the 1920s that quantum theory showed how this happened: atoms tended to seek to be in the lowest energy state, and sharing electrons could make the collective energy of two atoms lower than the sum of their respective energies when they were separate.
Our understanding of chemical bonds today is largely due to Nobel laureate American chemist Linus Pauling. In addition to explaining the causes of covalent bonds, Pauling also showed that in some cases, electrons are transferred from one atom to another, making the former positively charged and the latter negatively charged, and the two stick together in a way that electrostatic attraction forms an ionic bond. Then there are the metal bonds: some of these electrons are separated from their atoms to form a free "electron glue", and the metal atoms that have lost their outer electrons become positive ions, embedded in the "electronic glue", and rely on the electrostatic action with these electrons to bind to each other.
All of these chemical bonds either share electrons or transfer electrons. But even then, this simple classification was exhausted. Take van der Vals, for example. It can make molecules or atoms stick to each other, but it doesn't form conventional chemical bonds. Van der Vall forces help inert gases like helium and argon liquefy at very low temperatures. Sometimes it's also strong enough to chain atoms to a collective, such as two oxygen molecules (O) forming an O cluster of atoms through van der Waals force. But is van der Waals force a chemical bond? No one can give a definitive answer, because there has never been a consensus on what chemical bonds mean.
Considering that the concept of chemical bonds has evolved to this day, it is becoming more and more obscure, in this article we will refer to all the "bonds" that help molecules and atoms to be combined. So, the following is to introduce four alternative "chemical bonds" in addition to van der Waals force.
First, the dollar price key like a donkey and a horse
Atoms can share electrons in a local way, such as covalent bonds; they can also share electrons in a collective way, such as metal bonds; but it is impossible to combine both. So traditionally, covalent bonds and metal bonds have been considered incompatible.
But things may not be that simple. In 2019, scientists discovered a whole class of new materials with chemical bonds that resemble donkey-like horses: both covalent and metal bonds. Such materials are typically made up of a combination of metal-like elements (such as tellurium and germanium) from the junction of metals and non-metals, and elements located at the rightmost edge of the metal region of the periodic table (such as lead and tin). The bonds of these elements are combined, each consisting of two electrons (like covalent bonds), but these electrons are not owned by a single molecule, but are shared by the entire piece of material (like metal bonds). Scientists call this chemical bond "metavalent bond", and the resulting compound is called "primary metal".
The properties of nascent metals are also donkey-like horses. Normal metals are both good conductors of heat and good conductors of electricity, while primary metals, although they are good conductors of electricity, have poor thermal conductivity. What's more, its conductivity is particularly sensitive to ambient temperature. This makes primary metals a useful thermoelectric material that can be used to collect waste heat and convert it into electrical energy.
Second, the ghost hydrogen bond
In fact, before the van der Vals force, the first chemical bond to be found to be illogically defined was the hydrogen bond.
Hydrogen bonds involve hydrogen atoms that have formed covalent bonds with atoms such as oxygen, nitrogen, or fluorine. Since the atoms of these elements tend to monopolize the pair of shared electrons, so that the hydrogen carries a slight positive charge, hydrogen can attract other negatively polar molecules (such molecules, although the total charge is zero, are slightly negatively charged due to uneven charge distribution). The molecules are bound together by the medium of hydrogen, so it is called "hydrogen bond". But hydrogen bonds are usually much weaker than covalent bonds and ionic bonds, and they are not formed by sharing or transferring electrons, so they are not chemical bonds in the strict sense.
The presence of hydrogen bonds is widespread. Water (HO) at room temperature is liquid rather than gas, relying on hydrogen bonds between water molecules; molecular chains of amino acids form complex-shaped proteins through hydrogen bonds; pairs of bases on the structure of the DNA double helix, through which hydrogen bonds form base pairs...
However, people are still not entirely clear about what hydrogen bonds are. As a simple model of hydrogen bonding, a bifluoride ion (HF) is generally thought of as a covalent hydrogen fluoride (HF) molecule that is formed by binding a hydrogen bond to a fluorine ion, if the covalent bond is used to represent the covalent bond,... Represents a hydrogen bond, whose structural formula is F-H... F。 But scientists later discovered that when the bifluoride ions vibrate in water, their structure is in F-H... F and F... H... There is constant change between F. In other words, one of the bonds always switches between the covalent bond and the hydrogen bond. Here, the distinction between hydrogen bonds and covalent bonds begins to blur.
Keys are usually not rigid, but can be bent, stretched, and vibrated. This suggests that to determine the existence of a bond, we need to evaluate not only whether the atoms stick together, but also how long they have been glued. A typical example is supercritical water: water is heated beyond its critical point (at 218 atmospheres, the boiling point of water is 374 °C), where there is no longer a liquid and a gaseous state. People have been debating whether there are still hydrogen bonds at this time. Recently, a German chemist did a simulation of this. The results showed that the hydrogen bonds in the supercritical water broke very quickly. In this case, whether it is still eligible to become a hydrogen bond can only be said separately.
A better understanding of hydrogen bonds in supercritical water is of great help to the chemical industry. Supercritical water can dissolve what ordinary water cannot dissolve, and it is an environmentally friendly alternative to toxic organic solvents such as benzene or toluene.
Third, mechanical keys that do not involve electronics
By chaining the two key rings together, they are also "combined" into one. But this "combination" does not involve any force, but is only mechanically strung together.
There are also cyclic molecules in organic chemistry, such as benzene. They can also be mechanically strung together and connected permanently without involving electrons. Such a combination is called a "mechanical bond".
In this way, ring-like and chain-like molecules can be assembled into nano-machines. For example, by mimicking wheels with alkane molecules, or mimicking axles with chain hydrocarbon molecules, you can assemble nanoscale machines like Lego toys. These nanomachines can be used as switches and rotors, and their potential applications include molecular information storage, as well as artificial molecular muscles. The 2016 Nobel Prize in Chemistry was awarded to three chemists working in this field in recognition of their contributions to the design of molecular machines.
Until now, most of these molecular machines have been in separate formations in solution, and one challenge now is how to make these molecular machines work together. For example, the groups of them will only really work when many of these molecular machines work together.
Fourth, it is not a chemical bond formed by electrons
So far we are talking about chemical bonds, except for mechanical bonds, all involve electrons. So, can the bond be replaced by another particle? The answer is yes.
Electronic has a fat cousin called μ meson. μ mesons have the same negative charge as electrons, but are 207 times more massive and can also combine atoms. μ can be made in particle accelerators and can knock electrons out of atoms, replacing them. In this way, the place where the key was originally formed became the key μ. However, μ is unstable, and the existence time is only a fraction of a second, so the life of the μ key is also very short.
This doesn't sound very useful. But since μ are much heavier than electrons, the μ bonds will also be much stronger, which will bring the two nuclei in the molecule closer together. In an H molecule formed by μ bonds, the distance between the two hydrogen nuclei is 1/196 of the original. Researchers have been trying to exploit this effect by pulling the two hydrogen nuclei closer enough so that they can undergo a fusion reaction that releases nuclear energy.
In 1957, physicists demonstrated this idea for the first time with the heavy isotopes of hydrogen, deuterium and tritium. Although the demonstration was successful, unfortunately, such a nuclear reaction could not be sustained. "Continuous" means that after one reaction is completed, it can immediately activate another, similar to the chain fission reaction of the atomic bomb. Therefore, it is not possible to develop nuclear energy in this way.
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Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
Source: Big Tech
Edit: Garrett