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Select a polarisation mechanism to view

Within each atom or ion there is a positively charged nucleus surrounded by a negative electron cloud.

E polarisation + charge −qcentred here nucleus hascharge +q r

To undergo ionic polarisation, a material must have an ionic structure i.e. it must be composed of cations and anions.


You may find it helpful to consider the simple case of a cation and anion, of equal and opposite charges, held together by an ionic bond. This ion pair already possesses a dipole moment before the application of an electric field, however the sum of dipole moments over the entire material (i.e. the net polarisation) may still be zero.

+ +q -q r    μ = qr E

To gain a more general understanding of ionic polarisation, we must consider many ions at once.

+ E polarisation

Some molecules possess a permanent dipole - there is always a separation of charge within the molecule, even in the absence of an electric field. The example shown here is H2O.
Note that such permanent dipoles may also exist within ionic structures, e.g. crystals in which the cations are off-centred within the unit cells and do not coincide with the centre of negative charge.


Consider a material containing many such dipoles. In naturally non-polar materials like water, the dipoles are randomly arranged as a result of thermal motion. Therefore the dipole moments from different molecules cancel out and the net polarisation is zero.

δ+ δ+ δ− polarisation E