In Section 1.3, we saw that for molecules with one covalent bond, the dipole moment of the bond is identical to the dipole moment of the molecule. For molecules that have more than one covalent bond, the geometry of the molecule must be taken into account because both the magnitude and the direction of the individual bond dipole moments (the vector sum) determine the overall dipole moment of the molecule. Symmetrical molecules, therefore, have no dipole moment. For example, let’s look at the dipole moment of carbon dioxide (CO2 ). Because the carbon atom is bonded to two atoms, it uses sp orbitals to form the C−O σ bonds. The remaining two p orbitals on carbon form the two C−O π bonds. The individual carbon–oxygen bond dipole moments cancel each other—because sp orbitals form a bond angle of 180°—giving carbon dioxide a dipole moment of zero D. Another symmetrical molecule is carbon tetrachloride (CCl4). The four atoms bonded to the sp3 hybridized carbon atom are identical and project symmetrically out from the carbon atom. Thus, as with CO2 , the symmetry of the molecule causes the bond dipole moments to cancel. Methane also has no dipole moment.
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