To understand the dipole moment, one has to know about the covalent bond when molecules are formed by sharing of electrons, the bond between the two atoms is known as the covalent bond and a compound of two or more elements formed by covalent bonds is known as covalent compound. The halogens form stable diatomic molecules by sharing electrons one from each atom.
1. In a fluorine molecule, two atoms of fluorine are joined together by covalent bond. In a compound like water, which is formed by sharing of electrons, the oxygen atom shares its one electron with one hydrogen atom and it's another electron with another hydrogen atom forming an octet of electrons around oxygen and duet (two electrons), the configuration of He, around hydrogen. Such a compound is an example of a covalent compound. There are many other compound which form covalent bonds like; ammonia, ethylene, acetylene, hydrogen peroxide, hydrogen sulfide, carbon dioxide, carbon monoxide, nitrous oxide, phosphorus trichloride etc.
2. The covalent bond may not be symmetrically disposed towards the two atoms which are linked together. The atom towards which the duplet (two-electron pair) has moved becomes negatively charged and the other atom positively charged and the other atom positively charged, thus imparting polarity to the molecule. Such molecule or bonds are known as polar.
3. A chemical dipole arises due to unequal charge distribution between two atoms in a molecule. In other words, there is a charge separation of positive and negative charges within the dipole itself. Such condition may result in a diatomic molecule being polar became of difference in the electro-negativities of two atoms. This produces a dipole moment in a molecule.
The dipole moment is equal to the product of the magnitude of the charge difference and the ~ distance between the two centers.
- The magnitudes of dipole moment are expressed in Debye units named after the Dutch physical chemist, P. W. Debye. 1 Debye or 1D being equal to 1x10-16 electrostatic units (esu).
- When the bonding exists between two identical atoms, as in the case of chlorine molecule where both the atoms are identical and the centers of positive and negative electric charges coincide in it.
- This results in an equal attraction for the bonding electrons by the two atoms of chlorine. The bonding existing between two identical atoms as in case of chlorine molecules is called a homopolar bond.
The following generalization explains the existence or non-existence of dipole moment.
- Molecules made up of only one kind of atoms such as chlorine, phosphorous etc, do not have dipole moment.
- Spherically symmetrical molecules such as carbon tetrachloride, do not have dipole moments because all the atoms bonding to the central atom are of one type, and the dipole moment of each bond is cancelled by the dipole moment of another bond directed in the opposite direction.
- In a linear molecule like carbon dioxide the dipole moment is zero Whereas a non-linear molecule like water has large dipole moment of 1.844D.