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
• 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

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