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dipole moment of polyatomic molecules

dipole moment of polyatomic molecules - FIGURE I3.5(a The...

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Unformatted text preview: FIGURE I3.5 (a) The charge distribution in the water mol- ecule. (b) The water molecule in an electric field. (c) The electrostatic potential diagram of the water molecule. FIGURE I3.6 (a) The structure and charge distribution of the ammonia molecule. The polarity of the N—H bonds occurs because nitrogen has a greater electronegativity than hydrogen. (b) The dipole moment of the ammonia molecule oriented in an electric field. (c) The electrostatic potential diagram for ammonia. FIGURE I3.7 (a) The carbon dioxide molecule. (b) The opposed bond polarities cancel out, and the carbon dioxide molecule has no dipole mo— ment. (c) The electrostatic potential diagram for carbon dioxide. I33 Bond Polarity and DipoLe Moments 60 I 6+ 2? 13+ 5+ (:1) (b) o— 38‘ A- 5"' 3* a A+ 3+ @— {a} (b) (C) 3_ 25+ 5_ {a} {b} {C} Fig. 13.5 (a). This charge distribution causes the water molecule to behave in an electric field as if it had two centers of charge—one positive and one negative—as shown in Fig. 13.5 (b). Thus the water molecule has a dipole mo- ment. Similar behavior is observed for the NH3 molecule (Fig. 13.6). Some molecules have polar bonds but do not have a dipole moment. This occurs when the individual bond polarities are arranged in such a way that they can- cel. An example is the CO; molecule, a linear molecule that has the charge distribution shown in Fig. 13.7. In this case, since the opposing bond polari- ties cancel, the carbon dioxide molecule does not have a dipole moment. There is no preferential way for this molecule to line up in an electric field. (Try to find a preferred orientation.) There are many cases where the bond polarities in molecules oppose and exactly cancel each other. Some common types of molecules with polar bonds but without dipole moments are shown in Table 13.4. Example 13.2 For each of the following molecules, show the direction of the bond polari- ties. Also indicate which ones have dipole moments: HCI, C12, 803 (planar), CH4 (tetrahedral), and H28 (V-shaped). ...
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