Geometry—VSEPR

The shape of a molecule has quite a bit to do with its reactivity. This is especially true in biochemical processes, where slight changes in shape in three-dimensional space might make a certain molecule inactive or cause an adverse side effect. One way to predict the shape of molecules is the valence-shell electron-pair repulsion (VSEPR) theory. The 

Determine the number of electron-pair groups surrounding the central atom(s). Remember that double and triple bonds are treated as a single group. 

Determine the geometric shape that maximizes the distance between the electron groups. This is the geometry of the electron groups. 

For example, let s determine the electron-group and molecular geometry of carbon dioxide, C02, and water, H20. At first glance, one might imagine that the geometry of these two compounds would be similar, since both have a central atom with two groups attached. Let s see if that is true. 

Total Electron Pairs Electron- Group Geometry Bonding Pairs Nonbonding Pairs (Lone Pairs) Molecular Geometry 

---

Lewis Structures and Molecular Geometry VSEPR Theory Lewis Structures and Polarity... 

The tetrahedral geometry of methane is often explained with the valence shell electron pair repulsion (VSEPR) model The VSEPR model rests on the idea that an electron pair either a bonded pair or an unshared pair associated with a particular atom will be as far away from the atom s other electron pairs as possible Thus a tetrahedral geomehy permits the four bonds of methane to be maximally separated and is charac terized by H—C—H angles of 109 5° a value referred to as the tetrahedral angle... 

Although reservations have been expressed concerning VSEPR as an explanation for molecular geometries it re mains a useful too/for pre dieting the shapes of organic compounds... 

What IS the CNN geometry in each structure according to VSEPR" ... 

Apply the VSEPR method to deduce the geometry around carbon in each of the following species... 

Valence shell electron pair repulsion (VSEPR) model (Section 110) Method for predicting the shape of a molecule based on the notion that electron pairs surrounding a central atom repel one another Four electron pairs will arrange them selves in a tetrahedral geometry three will assume a trigo nal planar geometry and two electron pairs will adopt a linear arrangement... 

Table 1 6 VSEPR and Molecular Geometry Table 1 7 Dissociation Constants (pK ) of Acids Table 2 5 Oxidation Numbers in Compounds with More Than One Carbon...

VSEPR model, the dihalides of Be and Mg and the heavier halides of Ca and Sr are essentially linear. However, the other dihalides are appreciably bent, e.g. Cap2 145°, Srp2 -- 120°, Bap2 108° SrCl2 - 130°, BaCh - 115° BaBri -115° Bah 105°. The uncertainties on these bond angles are often quite large ( 10°) and the molecules are rather flexible, but there seems little doubt that the equilibrium geometry is substantially non-linear. This has been interpreted in terms of sd (rather than sp) hybridization or by a suitable id hoc modification of the VSEPR theory. ... 

R. J. Gillespie and I. Hargittai The VSEPR Model of Molecular Geometry, Allyn and Bacon, 1991. 

The major features of molecular geometry can be predicted on the basis of a quite simple principle—electron-pair repulsion. This principle is the essence of the valence-shell electron-pair repulsion (VSEPR) model, first suggested by N. V. Sidgwick and H. M. Powell in 1940. It was developed and expanded later by R. J. Gillespie and R. S. Nyholm. According to the VSEPR model, the valence electron pairs surrounding an atom repel one another. Consequently, the orbitals containing those electron pairs are oriented to be as far apart as possible. 

VSEPR electron-pair geometries. The balloons, by staying as far apart as possible, illustrate the geometries (left to right) for two to six electron pairs. 

---