Dipole Moments

An electric dipole is neutral overall, but has the positive and negative charges, +q and -q, separated by distance r; the  dipole aligns in an electric field. The dipole moment is defined by

   µ= qr     (1.1)

Heteronuclear diatomic molecules like CO have dipole moments because of the difference in electronegativity of the C and O atoms. CO molecules tend to align in an electric field, and this response is experimentally measurable.

In general, the bonds between dissimilar atoms in polyatomic molecules are polar, so we can speak of a bond dipole. The dipole moment of the molecule as a whole is the vector sum of the individual bond dipoles. Thus we can compute the dipole moment of CF₃Cl, with approximate 109.5° bond angles, in terms of the CF and C-Cl bond dipoles,

On the other hand, CF4, with four polar CF bonds has no net dipole moment, since

Molecules with unshared electron pairs on one side of the molecule, e.g., H₂O and NH₃, have larger dipole moments than would have been expected from bond polarity alone. The unshared electrons contribute significantly to the molecular dipole moment.