Introduction to polaring power and polarisability

Chemical bonds are broadly classed as covalent and ionic. In both cases the energy of the bonded atoms is lower than the energy of the separated atoms due to an electrostatic (positive to negative) attraction between the bonded atoms.

In covalent bonds the electrostatic attraction is between the bonding electrons and the nuclei and core electrons of both atoms.
In ionic bonds it is the attraction of the positive ion for the negative ion.

In fact, very few bonds are purely covalent or purely ionic.

For example the covalent OH bond in water is polarised due to the difference in electronegativity of hydrogen and oxygen.

Ionic bonds may have considerable covalent character if the cation is small and/or highly charged (high charge density).

These cations are said to be polarising ( or have high polarising power) because they distort the electron cloud of the anion, and the electron density between the nuclei is higher than in the corresponding ionic bond.

polarizing power of cations
Li⁺	Be²⁺	Al³⁺	O^2–	F^–
Na⁺	Mg²⁺	high	S^2–	Cl^–
K⁺	Ca²⁺		Se^2–	Br^–
Rb⁺	Sr²⁺		high	I^–
Cs⁺	Ba²⁺	polarizability of anions

The cartoon is an attempt to show that how the distortion of the electron cloud is greater for smaller cations.

Bonds between polarising cations and polarizable anions have significant covalent covalent character.
The polarizability of the anion depends both on its size and charge.

Anions of lower charge density are more polarizable.
For anions having the same charge, the larger anion is polarised to a greater extent (F^– < Cl^– < Br^– < I^–).
For anions of similar size, the more polarizable anion has a greater negative charge (S^2– > Cl^–).
This is because a larger charge indicates a larger excess of electrons over protons.

For polarisation (covalent character) to be significant for a bond, the cation must be polarising and the anion must be polarisable.

if the cation is strongly polarising, the anion must be polarisable
if the cation is moderately polarising, the anion must be very polarisable