Molecular Dipole Moments

Even though the total charge on a molecule is zero, the nature of chemical bonds is such that the positive and negative charges do not completely overlap in most molecules. Such molecules are said to be polar because they possess a permanent dipole moment. A good example is the dipole moment of the water molecule. Molecules with mirror symmetry like oxygen, nitrogen, carbon dioxide, and carbon tetrachloride have no permanent dipole moments. Even if there is no permanent dipole moment, it is possible to induce a dipole moment by the application of an external electric field. This is called polarization and the magnitude of the dipole moment induced is a measure of the polarizability of the molecular species.

Dipole Moment of Water

The asymmetry of the water molecule leads to a dipole moment in the symmetry plane pointed toward the more positive hydrogen atoms. The measured magnitude of this dipole moment is Treating this system like a negative charge of 10 electrons and a positive charge of 10e, the effective separation of the negative and positive charge centers is

This is 0.0039 nm compares with about .05 nm for the first Bohr radius of a hydrogen atom and about .15 nm for the effective radius of hydrogen in liquid form, so the charge separation is small compared to an atomic radius.

The polar nature of water molecules allows them to bond to each other in groups and is associated with the high surface tension of water.

Dipolar Bonding in Water

The dipolar interaction between water molecules represents a large amount of internal energy and is a factor in water's large specific heat. It also contributes to the fact that water has an unusually high boiling point. This dipolar interaction is an example of hydrogen bonding. The dipole moment of water provides a "handle" for interaction with microwave electric fields in a microwave oven. Microwaves can add energy to the water molecules, whereas molecules with no dipole moment would be unaffected.

The polar nature of water molecules allows them to bond to each other in groups and is associated with the high surface tension of water. The polar nature of the water molecule has many implications. It causes water vapor at sufficient vapor pressure to depart from the ideal gas law because of dipole-dipole attractions. This can lead to condensation and phenomena like cloud formation, fog, the dewpoint, etc. It also has a great deal to do with the function of water as the solvent of life in biological systems.