====== Chemical bonding ======

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

(see[[https://en.wikipedia.org/wiki/Chemical_bond#Strong_chemical_bonds|Wikipedia]])

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 : [[https://arxiv.org/pdf/2008.10219|arXiv:2008.10219 (cond-mat)  (2020)]] {{:oa_padlock_grn.png?16&nolink|Open Access}}

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 : [[https://onlinelibrary.wiley.com/doi/full/10.1002/adma.201806280|Advanced Materials Volume 31, Issue 3 1806280]] {{:oa_padlock_grn.png?16&nolink|Open Access}}

=== Recent progress on the Covalent bond ===

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: [[https://www.nature.com/articles/s41586-024-07965-1.epdf?sharing_token=rlTlAIng7l5lyq3fcC8JOtRgN0jAjWel9jnR3ZoTv0PH0EgAXRuPQ6W1_XfQNJNvkpuiFL9Pot1zElBZuVc_OvuC9lF2i82-e4xOIgMp3h9ain5dxF7W8Yg8NNhDHPuXZrQiUF5XQOCSU06LRfL9TWKKIgixcoPzN0HjSRZPjpiEAPeaGKcWMZaH6euOfQp3bAC7lnOBSffeHkbZNUCBvF2R2Lq0S47Ba00ZIQ4tfBo%3D|Direct evidence for a carbon–carbon one-electron σ-bond]] {{:oa_padlock_grn.png?16}}//Nature// (Sept. 2024).

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