Atomic Polarisability

Although the atom as a whole is electrically neutral, here there is a positively charge core (the nucleus) and a negatively charged electron cloud surrounding it. When the neutral atom is placed in an electric field E, these two regions of charge within the atom are influenced by the field. The nucleus is pushed in the direction of the field, and the electron the opposite way. Their centers of gravity tend to be separated along the direction of the field. This tendency is opposed by the electrostatic attraction between the two kinds of charges, until a state of equilibrium is reached. Thus the atom is polarized, with plus charge shifted slightly one way, and minus the other. The atom now has a tiny dipole moment p which points in the same direction as E. This induced dipole moment is proportional to the field:

                P = αE.                                … (1)

The constant of proportionality α is called the atomic polarizability. Induced dipole moment is also developed in molecules containing more than one atom when they are placed in electric field. The molecular polarizability α is also defined by the equation p = αE, where p is the induced dipole moment of the molecule. For a completely symmetrical molecule Eq. (1) takes the form of the most general linear relation between E and p.

            Px = αxxEx + αxyEy + αxzEz}
            Py = αyxEx + αyyEy + αyzEz}
            Pz = αzxEx + αzyEy + αzzEz}                        … (2)

The set of nine constants αij constitute the polarizability tensor for the molecule.

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