Lewis Atomic Model (1902 AD)

Also called “Dot and Dash Diagram”, “Valence Diagram”, “Lewis Diagram”, “Cubic Atom Model” or “Octet Rule”.

Gilbert N. Lewis proposed a graphical representation of the structure of atoms or non-polar molecules distributed in the form of a cube, in whose eight vertices were the electrons.

His theory was published in 1916 with the aim of explaining the phenomenon of atomic valence and, despite the fact that it was quickly abandoned in favor of the quantum mechanical model based on the Schrödinger equation, it has historical importance thanks to his contribution. in understanding chemical bonds.

• Single covalent bonds are formed when two atoms share an edge (Structure C) and this results in the exchange of two electrons.
• Ionic bonds are formed by the transfer of an electron from one cube to another without sharing an edge (A structure).
• An intermediate state in which only one corner is shared (structure B) was also postulated by Lewis.

• Double bonds are formed by sharing a face between two cubic atoms. This results in sharing four electrons.
  

The cubic atom model could not explain triple bonds, because there is no way for two cubes to share three parallel edges. Lewis suggested that the pairs of electrons in atomic bonds have a special attraction, giving rise to a tetrahedral structure, where the new location of the electrons is represented by the dotted circles in the center of the thick edges. This allows the formation of a single link when sharing a corner, a double link when sharing an edge, and a triple link when sharing a face. He also explains free rotation around single bonds and the tetrahedral geometry of methane.

This model underwent an update by Irving Langmuir, in 1919, where he raised the "atom of the cubic octet".