Dipole | Research & Encyclopedia Articles

Dipole

An understanding of molecular dipoles provides insight into how molecules interact and helps to explain many of their properties.

A molecule possesses a dipole if it is electrically asymmetric, i.e. if the centers of the distributions of positive and negative charge within the molecule are different. When a molecule possesses a dipole, it is said to be polar. The dipole moment is equal to the magnitude of the equal and opposite charges multiplied by the distance between the centers of the charge distributions.

In a covalent bond between atoms with unequal electronegativities (i.e. having unequal attraction for electrons), the atom with the larger electronegativity will attract the electrons shared in the bonding molecular orbital more, and the shared electrons will spend more time near that atom. It will, consequently, have a partial negative charge. The less electronegative atom will have a partial positive charge since the shared electrons spend less time near it. Because there is a separation of charges, with a partial positive charge at one end of the bond and an equal negative partial charge at the other, a dipole results. Such a bond is called a polar covalent bond.

In a covalent diatomic molecule, a difference in the electronegativity of the atoms will result in a dipole moment. The larger the difference in the electronegativities of the atoms, the larger the dipole moments. For instance, electronegativities in the halide sequence fluorine, chlorine,bromine, and iodine decrease from fluorine through iodine. The dipole moments of the hydrogen-halide sequence HF, HCl, HBr, and HI are 1.98, 1.03, 0.79, and 0.38 respectively.

A polyatomic molecule with bonds which are polar is not necessarily polar itself. In carbon dioxide (CO₂), for instance, the bonds between each of the oxygen atoms and the central carbon atom are polar: oxygen is more electronegative than carbon and, therefore, has a partial negative charge while the carbon atom has a partial positive charge. CO₂ is however, as shown in the first figure, a linear molecule so that, in effect, its two ends (oxygen atoms) have partial negative charges with the center of negative charge distribution in the center of the molecule, precisely at the point where the center of positive charge lies on the carbon atom. There is, as a result, no separation of charges and the CO₂ molecule is not polar. In water (H₂O) each of the bonds between a hydrogen atom and the central oxygen atom is polar with the oxygen atom having a partial negative charge and the hydrogen atoms having partial positive charges. The H₂O molecule is, however, non-linear so that the effective center of positive charge, halfway between the two hydrogen atoms, does not lie at the oxygen atom, which is the center of negative charge. Since there is a separation between the effective centers of charge, the ₂O molecule is polar and possesses a dipole.

The unit assigned to dipole moments is the debye. It is named to honor Peter Debye (1884-1966) who did extensive work with dipoles, developing methods for measuring their magnitudes and publishing, in 1912, a theoretical treatment of dipoles and their properties.

The measurement of the capacitances of various substances is a quantitative indication of their relative dipole moments. When polar molecules are placed between the parallel plates of a condenser, the molecules attempt to orient themselves so that their positive ends will be nearest the negatively charged plate (the cathode) with their negative ends nearest the positively charge plate (the anode). Even though thermal motion tends to disrupt this preferred orientation of the dipoles, a sufficient number of molecules, on the average, are oriented in this manner to cause the capacitance of the system to be increased. The molecules with higher dipole moments have higher capacitances. They are called dielectrics. Substances with low capacitance are insulators. The dielectric constant of a substance is the ratio of the capacitance of a pair of condenser plates with the substance between it and the capacitance of the plates in a vacuum.

When dipolar molecules are adjacent, they are attracted to each other through dipole-dipole forces. These forces result from the fact that the negative end of one dipole is attracted to the positive end of another dipole. This attraction leads to a more stable arrangement of molecules. Consequently, upon solidification, molecules align themselves to take maximum advantage of the dipole-dipole forces of attraction. In liquids, the molecules are also oriented in this way but they do not maintain the specific arrangement because the molecules are in constant motion. Additional energy is necessary to break apart molecules that are held in stable relationships with their neighbors because of the mutual attractive force between dipoles, and therefore, the boiling and melting points of materials whose molecules have dipoles are generally higher than they would be if there were no dipoles. For example, the boiling point of hydrochloric acid, HCl, a polar diatomic molecule is about -64°F (-53°C, 220 K). In contrast, the boiling point of argon (a non-polar fluid of atoms with nearly the same molar mass) is about -280°F (-173°C, 100 K). HCl has a higher boiling point because of its polar nature.

Molecules may change their rotational energy levels by absorbing energy from electromagnetic radiation in the microwave region of the spectrum. In order for a molecule to interact with and absorb radiation it must possess a dipole. As a result, such symmetrical non-polar molecules as CO₂, CH₃Cl, and N₂ cannot change their rotational energy by absorbing electromagnetic radiation. On the other hand, asymmetrical molecules such as HCl, HCN, and H₂0, that have dipole moments, absorb radiation and exhibit absorption spectra in the microwave region.