Molecular dipoles

A dipole moment is created when there is a difference in electronegativity between the bonded atoms. Thus,  electrons from one atom are closer attracted to the other, causing one atom to become Δ+, and the other Δ-. Like the name would suggest, a dipole molecule has two (electrical) poles, while a nonpolar molecule is electrically completely neutral and “featureless”. The first scientist to study molecular dipoles, was the chemist Peter J. W. Debye, so that now, dipole moments are measured in units named “debye” in his honor.

For molecules there are three types of dipoles:

  • Permanent dipoles-  occurring  when two atoms in a molecule have substantially different

Electronegativity (one atom attracts electrons more than another, becoming more negative, while the other atom becomes more positive)

  • Instantaneous dipoles- ocurring  due to chance when electrons happen to be more concentrated in one place than another in a molecule, creating a temporary dipole.
  • Induced dipoles- occuring when one molecule with a permanent dipole rejects another molecule’s electrons.

More generally, an induced dipole of any polarizable charge distribution ρ (remember that a molecule has a charge distribution) is caused by an electric field external to ρ. This field may, for instance, originate from an ion or polar molecule in the vicinity of ρ or may be macroscopic (e.g., a molecule between the plates of a charged capacitor). The size of the induced dipole is equal to the product of the strength of the external field and the dipole polarizability of ρ.

Dipole moment values are usually obtained from measuring the dielectric constant. Some typical gas phase values in debye units are:

  • carbon dioxide: 0
  • carbon monoxide: 0.112 D
  • ozone: 0.53 D
  • phosgene: 1.17 D
  • water vapor: 1.85 D
  • hydrogen cyanide: 2.98 D
  • cyanamide: 4.27 D
  • potassium bromide: 10.41 D

 KBr (a ionic compound made from a metal and a non-metal) has one of the highest dipole moments because it is a very ionic molecule (which only exists as a molecule in the gas phase).


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