Polarisation of ions: when two charged ions (cation and anion) of the ionic molecule come closer to each other, the cation attracts the electron cloud of the outermost shell of the anion towards itself & hence the symmetrical Shape off the anion get distorted (i.e, deformed or polarized). This phenomenon is called Polarisation of anion.
Note:
The ability of a cation to polarise and anion is called its polarising power or polarising ability.
The tendency of an anion to get polarized by a cation is called its polarisability.
Factor affecting the magnitude of polarising power of a cation
Charge on cation: Higher the charge, higher is the polarizing power of cation.
Size of cation: Smaller the size, higher the polarizing power of cation, higher will be the covalent character in ionic bond. eg
Electronic configuration of cations: if cations have same charge same and almost same size, then cations having inert gas configuration (ns^2 np^6), have less polarising power than the cations having pseudo inert gas configuration. ( transition metal cations, ns^2 p^6 d^10).
Question No.1
percentage covalent character in CuCl, is higher as compared to NaCl, explain?
Ans: In CuCl, Cu+ has pseudo inert gas configuration (ns2 p6 d10) while Na+ (in NaCl) has inert gas configuration (ns2 np6).
Polarising power of Cu+ Cation is higher than of Na+ cation and hence electrons cloud of chlorine in CuCl is more polarised as compared to chlorine in NaCl.
According fjan’s rule more polarisation of anion, more is the covalent character in ionic molecule. Therefore, that is why the CuCl molecule has more covalent character than NaCl.
*Important to Note: Although Hg2+ (5s^2 p^6 d^10) and Zn2+ (3s^2 p^6 d^10) have same charge and nearly same the same size, yet Hg2+ has more polarising power than Zn2+.
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Factors affecting the polarisability of an anion:
(1) Charge on anion: As magnitude of negative charge on anion increases, the polarisability of anion also increases i.e. higher the negative charge on anion higher is the polarisibility of anion.
Negative charge on anion (D.P) Polarisability of anion (D.P=Directly Proportional)
(2) Size of anion: As the size of anion increases, the polarisability of anion increases.
Size of anion (D.P) polarisability of anion (D.P=Directly Proportional)
Question No.2
Arrange the following anions in increasing order oftheir polarisability F^-, Cl^- , Br^- , I^- ?
Soln: Larger size of anion, higher will be polarisability of anion. So iodide has the largest size among halides, will has highest polarisability and fluoride (F^- ) has the smallest size, and has the lowest polarisability.
F^- < Cl^- < Br^- < I^-
Fajan 's Rule
According to Fajan. higher the polarising power of cation and higher polarisibility of anion, more will be the covalent character in ionic molecule.
* If ionic molecules follow the following conditions, will have more covalent character.
(i) Higher +ve charge on cation
(ii) Small size of cation.
( iii) The cation having pseudo inert gas configuration(ns^2 p^6 d^10) have more polarising power than that of cations. having inert gas configuration (ns^2 p^6).
Reason: Noble gas configuration is the most effective at shielding the nuclear charge, so ions without the noble gas configuration will have high charges at their surfaces, and thus be highly polarising.
( iv ) anion should have high negative charge.
(v) The anon should be large in size.
Note: With the help of Fajan's rule number of properties like melting point, covalent & ionic character, solubility etc. of conpound/molecule can be explained.
In an another blog post we will cover the Applications of fajan's rule, So stay tune for that and subscribe to our website.
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