Polarity
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
ص411-412
2025-12-02
54
Polarity
A polar molecule is one with a permanent electric dipole moment (HCl, O3, and NH3 are examples). If the molecule belongs to the group Cn with n > 1, it cannot possess a charge distribution with a dipole moment perpendicular to the symmetry axis because the symmetry of the molecule implies that any dipole that exists in one direction perpendicular to the axis is cancelled by an opposing dipole (Fig. 12.13a). For example, the perpendicular component of the dipole associated with one O-H bond in H2O is cancelled by an equal but opposite component of the dipole of the second O-H bond, so any dipole that the molecule has must be parallel to the twofold symmetry axis. However, as the group makes no reference to operations relating the two ends of the molecule, a charge distribution may exist that results in a dipole along the axis (Fig. 12.13b), and H2O has a dipole moment parallel to its twofold symmetry axis. The same remarks apply generally to the group Cnv, so molecules belonging to any of the Cnv groups may be polar. In all the other groups, such as C3h, D, etc., there are symmetry operations that take one end of the molecule into the other. Therefore, as well as having no dipole perpendicular to the axis, such molecules can have none along the axis, for otherwise these additional operations would not be symmetry operations. We can conclude that only molecules belonging to the groups Cn, Cnv, and Cs may have a permanent electric dipole moment. For Cn and Cnv, that dipole moment must lie along the symmetry axis. Thus ozone, O3, which is angular and belongs to the group C2v, may be polar (and is), but carbon dioxide, CO2, which is linear and belongs to the group D∞h, is not.
Fig. 12.13 (a) A molecule with a Cn axis cannot have a dipole perpendicular to the axis, but (b) it may have one parallel to the axis. The arrows represent local contributions to the overall electric dipole, such as may arise from bonds between pairs of neighbouring atoms with different electronegativities.
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