Representation of Lewis Structures

Lewis and Kossel s proposals coincided with the shell structure of atoms which resulted firom the hybrid classical/quantum model for the hydrogen atom developed by Bohr [37, 38] and subsequently extended by Sommerfeld [39 1] to other atoms. They did not fully appreciate the physical imphcations of a quantum model. Specifically Lewis based his model on the following postulates  

Kernel electrons (or core electrons in closed shells) remain unaltered in all ordinary chemical changes. 

An atom in a molecule tends to hold an even number of electrons in its valence shells. 

Electrons in shells which lie outside the kernel are mutually interpenetrable, and their pairing leads to the formation of a covalent bond. 

Lewis and Kossel both suggested the electrons in molecules and ions form concentric groups of either two or eight electrons, although they represented them in quite different ways. Lewis preferred to represent them using a cubic model (his static representation of the electrons led to a symmetrical arrangement if they were located at the vertices of a cube), whereas Kossel preferred to use concentric rings to illustrate the successive shells. The different representations are summarised for neon in Fig. 1. 

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Fig. 3 Representation of Lewis structures based on the attainment of closed shells by electron-pair sharing. The initial dot structures have been progressively replaced by line structures to represent the two-electron two-centre bonds. Dots are only retained when they have stereochemical consequences or are required to represent organic reactions using the curly arrow notation (see Fig. 4)...

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