CHAPTER 1 Stereochemistry and Bonding in Main Group Compounds: 15 Figure 6. Structure of SO 2 molecule from the VSEPR model. 3. Four electron-pair domains: i) CH 4 : The central atom in CH 4 molecule is C which has four valence electrons (2, 4). Now because each hydrogen atom needs one electron to complete its duplet (1), the C atom uses its all four valence electrons in sharing to create four bonding one-electron-pair domains only. Hence the geometry for minimum repulsion will be perfect tetrahedral and the normal bond angle will be 109°28ґ. Figure 7. Structure of CH 4 molecule from the VSEPR model. ii) NH 3 : The central atom in NH 3 molecule is N which has five valence electrons (2, 5).
Now because each hydrogen atom needs one electron to complete its duplet (1), the N atom uses its three valence electrons to create three bonding one-electron-pair domains, while two electrons are left as lone pair domain. Now though the geometry for four electron-pair domains is tetrahedral with 109°28ґ, in this case, the non-bonding domain would require more space than the bonding domains. This, in turn, would result in greater lone-pair–bond-pair repulsion yielding a pyramidal-shaped geometry with H−N−H bond angle (107.8°) slightly less than the normal 109°28ґ of a perfectly tetrahedral system. Figure 8. Structure of NH 3 molecule from the VSEPR model.
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