Lewis structure essential features

Lewis s theory of the chemical bond was brilliant bul it was little more than guesswork inspired by insight. 1 ewis had no way of knowing why an electron pair, the essential feature of his approach, was so important. The valence-bond theory explained this point but could not explain the properties of some molecules. For example, the Lewis description of 02 is 0=0, with all the electrons paired. However, oxygen is a paramagnetic substance (Fig. 3.28 and Box 3.2), and paramagnetism is a property of unpaired electrons. The magnetism of O, therefore contradicts both the l wis structure and the valence-bond description of the molecule. 

The Lewis structure notation is useful because it conveys the essential qualitative information about properties of chemical compounds. The main features of the chemical properties of the groups that make up organic molecules, ... 

However, we can recognize that the essential feature of a Lewis structiu-e is the localized 1-c, 2-c bonding pattern rather than electron pairing per se. Moreover, the a and 3 electrons of open-shell species necessarily experience different Coulomb and exchange forces and may hence lead to different spatial distributions, spin orbitals, and localization patterns. The familiar open-shell concept of different orbitsds for different spins can therefore be extended to a different Lewis structures for different spins (DLSDS) picture, where as usual we associate a Lewis structure with a specified pattern of one-center and two-center (spin-)NBOs. Because the NBO... 

In a sense, the various apparent Lewis structural exceptions considered in this chapter all serve to blur the boundaries between molecular and supramolecular domains. The topics of this chapter therefore infringe on the intermolecular interactions of Chapter 9, where H-bonded species will be considered. These exceptions also force further recognition of intra- and intermolecular fractional bonding that is the essential feature of the resonance extension of primitive Lewis structure concepts (Chapter 5). 

In this section we combine the ideas introduced in the preceding three sections with a few new concepts to write a variety of Lewis structures. Let us begin with a reminder of some of the essential features of Lewis structures that we have already encountered. 

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