Polarisation occurs when the electron cloud of an atom or ion is distorted by the influence of a nearby positive atomic nucleus or electric field. Polarisability is the ability of an atom or ion to become polarized. Polarization can range from a slight distortion to a sufficient distortion that forms a covalent bond between ions. According to Fajans' Rules, ions with higher charges and smaller sizes polarize anions more, while ions with higher charges and larger sizes polarize cations more. Ions with partially filled d orbitals also polarize more than ions with noble gas configurations.

1. Polarisation and Polarisability

2. Polarisation and Polarisability
Polarisation- The negative cloud of electrons is
distorted due to influence of a positive atomic nuclei.
Polarisability- ability of atom/ion to be distorted due
to influence of electric field such that of an atom.

3. + -
Ideal ion pair with no polarisation
- - - - - - - - Dashed lines represent hypothetical unpolarized ions

4. +
-
Mutually Polarised ion Pair

5. +
-
Polarisation sufficient to form a covalent bond

6. Fajans Rules
• High charge and small size cation exert greater polarisation in
anions than lower charged cations.
(Li + 76 pm) (Mg2+ 72 pm)
• High charge and large size anion experience greater polarisation by
the cations. (I- 206 pm) (Se2- 198 pm) (As3- 222 pm)
• Electronic configuration with partially filled d electron will be more
polarising than ions having noble gas configuration.
Kazimierz Fajans