Concept of Polarization





Application of the concept of Polarization

Polarization power of a cation is generally represented by \phi and is known as ionic potential or charge density. It can be represented as:

\phi = charge on cation / radius of cation.

The important applications of this concept are given below:

(i) Character of a cation : The larger the value of \phi, the greater is the degree of covalence and its tendency to form complex compounds.

(ii) Salvation energy : When a substance is introduced into a solvent the inter action that takes place is called salvation, and the energy change involved in this process is known as hydration and hydration energy.

The larger the value of \phi for a cation, the greater is its tendency towards salvation. For example, the value of \phi decreases form Li^+ \text{to} Cs^+ in 1st group, therefore lithium forms hydrated compounds such as LiCl.2H_2O LiClo_4 . 3H_2O etc. while other alkali metal ions do not form hydrates. This is the reason that liCl is soluble in organic solvents (pyridine etc). while other halides of 1st group are insoluble in organic solvents.

(iii) Diagonal relationship : Although the diagonal relationship cannot be full explained by the value of \phi , even then it is very helpful in some examples e.g. the values of \phi for Be^{2+} \text{and} Al^{3+} are 6.4 and 6.0 respectively; therefore these elements exhibit diagonal relationship.

(iv) Nature of oxides : Covalent character of the M – O bond increases with larger value of \phi for the cation, (M) and at the same time the oxide will be acidic in nature. for example, covalent and acidic character is of the order: Na_2O < MgO < Al_2O_3, while their values of \phi are of the order:

\phi_{Na}^+ (1.05) < \phi_{Mg}^{2+} (3.08) < \phi_{Al}^{3+} (6.0) \\ na_2 is highly basic in nature while Al_2O_3 is Amphoteric in nature.

According to Cartlidge, M_2O_n is basic when \sqrt{\phi_M^{n+}} < 2.2, is Amphoteric when \sqrt{\phi_M^{n+}} = 2.2 \text{to} 3.2 \text{and is acidic when} \sqrt{\phi_M^{n+}} > 3.2

(v) Nature of anhydrous halides : With  larger value of \phi for a cation, the anhydrous halides will be more covalent and nonconductor of electricity. If the value of \sqrt{\phi} for a cation is more than 2.2 the halide will be covalent and non-conductor of electricity. On the other hand, if it is less than 2.2 the halide will be ionic and good conductor of electricity.

(vi) Thermal stability of carbonates : With larger value of \phi for a bivalent cation, the less will be its thermal stability. With increases in the value of \phi there will be strong pull of the electron could of the neighbouring oxygen atom of the carbonate which thus gets readily decomposed ( \overset{0}{C}) e.g. BeCo_3 (~100) < MgCO_3(350) < CaCO_3(547) < SrCO_3(778) < BaCO_3(998).

Similarly reason may also be applied to the decomposition of sulphates, hydroxides, nitrates etc.



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