Molecular Dipole MomentsEven 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.
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Index Electric dipole concepts | ||
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Dipole Moment of Water
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. |
Index Voltage concepts Electric dipole concepts Reference Halliday, Resnick, Walker Sec 24.9 | ||
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Dipolar Bonding in Water
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.
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