The Shapes of Molecules Valence Shell Electron-Pair Repulsion Theory

9 The Shapes of Molecules Valence Shell Electron-Pair Repulsion Theory 

When two molecules approach one another to begin a chemical reaction, the probability of a successful encounter can depend critically on the three-dimensional shapes and the relative orientation of the molecules, as well as on their chemical identities. Shape is especially important in biological and biochemical reactions, in which molecules must fit precisely onto specific sites on membranes and templates drug and enzyme activity are important examples. Characterization of molecular shape is therefore an essential part of the study of molecular structure. 

FIGURE 3.17 Three-dimensional molecular structures of (a) H2, (b) SO2, (c) NH3, and (d) C2H4O2, showing bond lengths and angles. (Courtesy of Prof. Andrew J. Pounds, Mercer University, Macon, GA, and Dr. Mark A. Iken, Scientific Visualization Laboratory, Georgia Institute of Technology, Atlanta, GA.) 

FIGURE 3.18 The positions of minimum energy for electron pairs on a sphere centered on the nucleus of an atom. The angles between the electron pairs are indicated. For two, three, and four electron pairs they are 180°, 120°, and 109.47°, respectively. 

The Valence Shell Electron-Pair Repulsion Theory 

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