14 A Textbook of Inorganic Chemistry – Volume I Application of the VSEPR Theory to Predict Molecular Geometries The VSEPR theory can successfully be used to explain the qualitative geometrical profile of molecular species with coordination numbers ranging from two to seven. Some of the most common illustrative examples are given below. 1. Two electron-pair domains: i) BeCl 2 : The central atom in BeCl 2 molecule is Be which has two valence electrons (2, 2). Now because each chlorine atom needs one electron to complete its octet (2, 8, 7), the Be atom uses its both valence electrons to create two bond pair domains only.
Hence the geometry for minimum repulsion will be linear and the normal bond angle will be 180°. Figure 4. Structure of BeCl 2 molecule from the VSEPR model. 2. Three electron-pair domains: i) BF 3 : The central atom in BF 3 molecule is B which has three valence electrons (2, 3). Now because each fluorine atom needs one electron to complete its octet (2, 7), the B atom uses its three valence electrons to create three bond pair domains only. Hence the geometry for minimum repulsion will be trigonal and the normal bond angle will be 120°. Figure 5. Structure of BF 3 molecule from the VSEPR model. ii) SO 2 : The central atom in the SO 2 molecule is S which has six valence electrons (2, 8, 6).
Now because each oxygen atom needs two electrons to complete its octet (2, 6), the S atom uses its four valence electrons to create two bonding two-electron-pair domains, while two electrons are left as a lone pair domain. Now though the geometry for three electron pair domains is trigonal planar with 120°, in this case, the non-bonding one- electron-pair domain would require more space than the bonding domains. This, in turn, would result in a greater lone-pair–bond-pair repulsion yielding a V-shapes geometry with an actual bond angle slightly less than the normal 120° of a perfectly trigonal planar system.
However, the actual O−S−O bond angle is 119.3° which is still not very much less than ideal 120° as we expected it to be; this can be attributed to the larger electron density from the two-electron-pair nature of each bond pair domains. Buy the complete book with TOC navigation, high resolution images and no watermark. Copyright © Mandeep Dalal