Chemistry textbooks normally describe three types of chemical bonds - the strong attraction between atoms, ions or molecules that enables the formation of all chemical compounds.

• The Ionic bond
• The Covalent bond (see notes below)
• The Metallic bond

(seeWikipedia)

Despite more than 150 years of research, however, there's still not a full picture of exactly how the outermost 'layers' of electrons within atoms form the bonds.

In addition, recent research has identified a previously overlooked bond, which has been called the 'Metavalent' bond.

The chemical bond is one of the most powerful, yet controversial concepts in chemistry, explaining property trends in solids. Recently, a novel type of chemical bonding has been identified in several higher chalcogenides, characterized by a unique property portfolio, unconventional bond breaking and sharing of about one electron between adjacent atoms. Metavalent bonding is a fundamental type of bonding besides covalent, ionic and metallic bonding, raising the pertinent question, if there is a well-defined transition between metavalent and covalent bonding.

Source : arXiv:2008.10219 (cond-mat) (2020)

The newly discovered bond has highlighted the need for a more nuanced description of chemical bonds in general.

[…] we have provided evidence that metavalent bonding cannot be described by any combination of the three “textbook” mechanisms—it therefore constitutes a fourth fundamental bonding mechanism by accepted definitions. Our work opens up a conceptually new avenue for materials design: by searching for desired properties in a 3D space and then mapping this back onto the 2D plane of bonding, allowing scientists to navigate structural and composition spaces and to identify promising target materials.

Source : Advanced Materials Volume 31, Issue 3 1806280

The original idea for the Covalent bond, in which two atoms share an electron pair, which was initially suggested by G,N. Lewis in 1916 (ref. The atom and the molecule. J. Am. Chem. Soc. 38, 762–785)

Then in 1931, L. Pauling proposed the concept of covalent bonds with one unpaired electron (known as ‘one-electron σ-bonds’) which is shared between two atoms. (ref. The nature of the chemical bond. II. The one-electron bond and the three-electron bond. J. Am. Chem. Soc. 53, 3225–3237)

Since then, only a few practical examples have been found. Until a 2024 study discovered the first evidence for one-electron bonds between carbon atoms - suggesting that the bonds may be far more common than had been thought. For technical details, see: Direct evidence for a carbon–carbon one-electron σ-bond Nature (Sept. 2024).

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