The Lewis electron-pair model

The physical basis of the Lewis electron pair model... 

G.A. GALLUP, The Lewis electron-pair model, spectroscopy and the role of the orbital picture describing the electronic structure of molecules. 

The fact that the dihalides form stable molecules, that the monohalides are much less stable than the dihalides, and that tri- and higher halides are unknown, is, of course, in perfect agreement with the Lewis electron pair model. Each of the two atoms forming the bond contributes one electron to a bond pair that moves in the space between and around the two kernels X M X. 

We shall refer to a Group 13 atom that forms three single bonds as predicted by the Lewis electron pair model as Lewis-valent and to a Group 13 atom that forms just one single bond as subvalenl. M-Cl bond distances and bond dissociation energies of the gaseous... 

The fact that the Group 14 elements form four single bonds is in agreement with the Lewis electron pair model. In these compounds the central atom is surrounded by four bonding electron pairs and the structure is tetrahedral in agreement with the VSEPR model. The observation that the E-Cl bonds in the tetrachlorides are shorter than in the dichlorides is, however, difficult to rationalize in terms of the VSEPR model. 

He, somewhat mischievously, made the following comment on the relationship between his molecular orbital analysis and the Lewis electron-pair model "Now I have a favourite argument that Lewis electron pair bonding is better described by a pair of electrons in a molecular orbital than by the Heitler-London bond. If the chemical bond has any polarity, it is necessary to add an ionic term, that is a Heitler-London plus an ionic term, to represent the bond. That is rather a messy description whereas the molecular orbital- this is not the spectroscopic but the chemical molecular orbital, the delocalized molecular orbital fits very nicely to the... 

However, a descriptive outline of the theory is presented at the beginning of Section 3. Section 4 introduces the properties of the laplacian of the electron density distribution and its relation to the Lewis electron pair model. 

Other simplified quantum treatments, such as the Lewis electron pair and orbital overlap models, have proved useful in teaching and they give qualitative predictions of the structures of molecular compounds, but they become unwieldy when applied to solids. They have not proved to be particularly helpful in the description of the complex structures found in inorganic chemistry and have therefore not been widely used in this field. 

The shared-electron pair model introduced by G.N. Lewis showed how chemical bonds could form in the absence of electrostatic attraction between oppositely-charged ions. As such, it has become the most popular and generally useful model of bonding in all substances other than metals. A chemical bond forms when electrons are simultaneously attracted to two nuclei, thus acting to bind them together in an energetically -stable arrangement. The covalent bond is formed when two atoms are able to share a... 

The extent to which individual electron pairs are localized in distinct spatial regions has been carefully analyzed by Bader and Stephens (1975) using the minimum fluctuation criterion. These authors arrive at the conclusion that the model of spatially localized pairs is appropriate for LiH, BeH2, BH3, and BH-r, it is borderline for CH4, but in NHj, OH2, FH, Ne, N2, and F2, the motions of the valence electrons are so strongly inter-correlated, the localized pair model ceases to afford a suitable description. Moreover, their results provide no physical basis for the view that there are two separately localized pairs of nonbonded electrons in H20. This clearly shows the limit of the Lewis electron pair concept which otherwise has practically disappeared in Linnett s theory. 

Even though the nature of the bonding in the dimetallenes is well understood, the properties of digermenes and distannenes leave us in a terminological quandary should we describe the E-E bonds in these molecules as double and accept that a double bond may be weaker than a single bond between the same atoms, or should we describe the bonds as single and accept that Lewis electron pair model has failed ... 

The covalent, or shared electron pair, model of chemical bonding was first suggested by G N Lewis of the University of California m 1916 Lewis proposed that a sharing of two electrons by two hydrogen atoms permits each one to have a stable closed shell electron configuration analogous to helium... 

Figure 4.17 Triple bonds (a) Lewis model of two tetrahedra sharing a face, (b) three electron pair domains, and (c) end-on view of the three electron pair domains forming the triple bond.

Electron Pair Localization and the Lewis and VSEPR Models... 

Under the conditions of maximum localization of the Fermi hole, one finds that the conditional pair density reduces to the electron density p. Under these conditions the Laplacian distribution of the conditional pair density reduces to the Laplacian of the electron density [48]. Thus the CCs of L(r) denote the number and preferred positions of the electron pairs for a fixed position of a reference pair, and the resulting patterns of localization recover the bonded and nonbonded pairs of the Lewis model. The topology of L(r) provides a mapping of the essential pairing information from six- to three-dimensional space and the mapping of the topology of L(r) on to the Lewis and VSEPR models is grounded in the physics of the pair density. 

AIM theory provides a physical basis for the theory of Lewis electron pairs and the VSEPR model of molecular geometry. Equipped with computers and computer-generated, three-dimensional electron density maps, scientists are able to view molecules and predict molecular phenomena without even having to get off their chairs ... 

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