Dipole moment:

Dipole moment is defined as the product magnitude of charge on the atoms and the distance between the two bonded atoms. Its SI unit is columb meter. An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret. A magnetic dipole is a closed circulation of electric current. Dipoles can be characterized by their dipole moment, a vector quantity. The electric dipole moment would point from the negative charge towards the positive charge, and have a magnitude equal to the strength of each charge times the separation between the charges. For the current loop, the magnetic dipole moment would point through the loop with a magnitude equal to the current in the loop times the area of the loop. The only known mechanisms for the creation of magnetic dipoles are by current loops or quantum-mechanical spin since the existence of magnetic monopoles has never been experimentally demonstrated. Many molecules have such dipole moments due to non-uniform distributions of positive and negative charges on the various atoms. Such is the case with polar compounds like hydroxide (OH^-), where electron density is shared unequally between atoms.

A molecule with a permanent dipole moment is called a polar molecule. A molecule is polarized when it carries an induced dipole. There are three types of dipoles:

Permanent dipoles: These occur 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: These occur due to chance when electrons happen to be more concentrated in one place than another in a molecule, creating a temporary dipole.

Induced dipoles: These can occur when one molecule with a permanent dipole repels another molecule's electrons, inducing a dipole moment in that molecule.

When the symmetry of a molecule cancels out a net dipole moment, the value is set at 0. The highest dipole moments are in the range of 10 to 11. 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.

Even if the total charge on the molecule is zero, the nature of chemical bonds is such that the positive and negative charges do not totally overlap in most molecules. Such molecules are called polar because they have a permanent dipole moment. A fine example is the dipole moment of water molecule. Molecules having the mirror symmetry like nitrogen, oxygen, carbon dioxide, and carbon tetrachloride don’t have permanent dipole moments. Even though there is no permanent dipole moment, it is possible to induce a dipole moment by application of the external electric field. This is called as polarization and the magnitude of dipole moment induced is the measure of the polarizability of molecular species. The asymmetry of water molecule leads to the dipole moment in symmetry plane pointed toward the more positive hydrogen atoms. The dipolar interaction among water molecules represents the large amount of internal energy and is a factor in large specific heat of water. The dipole moment of water gives a "handle" for interaction with the microwave electric fields in the microwave oven. Microwaves can add energy to the water molecules, while molecules with no dipole moment would be unaffected. The polar behavior of water molecules allows them to bond with each other in groups and is associated with high surface tension of water. The polar nature of water molecule has several implications. It causes water vapor at sufficient vapor pressure to depart from ideal gas law due to dipole-dipole attractions. This can lead to condensation and phenomena such as cloud formation, fog, the dew point, etc.

The electric dipole moment for the pair of opposite charges of magnitude q can be defined as the magnitude of the charge times the distance in between them and the defined direction is toward positive charge. It is very useful concept in atoms and molecules where the effects of charge separation are measurable, but the distances in between the charges are too small to be measurable easily. It is a useful concept in dielectrics and other applications in the solid and liquid materials.