B.sc 1st year Book
Polarization of ions | Fajans Rule
Here it is the point to remember that as a cation polarizes into an anion, an anion also polarizes into a cation. The extent of polarization of cation by anion is much less and can be neglected, The polarization caused by anion to cation is suppressed by electrons around the cation. Hence, generally, the polarization caused by anion is not considered. Fajan Rule
Fajans Rules :
Postulates of Fajans Rule
(i) Size of cation :
Hence, in groups it has the following order:
Group IA Captions: Li+ >Na+ >K+ > Rb+ > Cs+ Group IA Captions: Be2+ >Mg2+ >Ca2+ > Sr2+ > Ba2+
The high magnitude of polarizing power of L+ and Be2+ ions makes their salts show maximum covalent character. It is evident by the minimum melting points of anhydrous chlorides of Be2+ ion than the other divalent cations of the 11 A group as shown below :
Chlorides of Group IIA: BeCl2 MgCl2 CaCl2 SrCl2 BaCl2 Melting Point (2C): 405 712 772 872 960
(ii) Charge on the cation :
Na+ < Mg2+ < Al3+ < Sn4+
With the increase of polarization of Cl2 anion by these actions, the covalent character between the cation and anion of their chlorides also increases as we move from NaCl to SnCl4. It is evident by a gradual decrease in the meeting points of their chlorides as shown below:
Chirodies of 3rd Period: NaCl MgCl2 AlCl2 Melting Point (C) = 800 712 575
(iii) Electronic configuration:
Related Topic | Chemical Bonding
|Chemical Bonding||Inert Pair Effect|
|Lattice energy||Fajans Rule|
|Covalent Bond||Coordinate Bond|
|Odd electron bond||Metallic bond|
|Hydrogen Bonding||M.O. Theory|
|Sidgwick-Powell theory||VSEPR theory|
|Hybridization of atomic orbitals|
Example Fajans Rule or Fajan Rule :
(i) CuCl(442∘C) < NaCl(800∘C) (iii) AgCl(455∘C) < KCl(776∘C) (iii) AuCl (170∘C) < RbCl(715∘C)
Factors that affect the Polarizability of anions by Fajan Rule :
1- Size of an anion :
F−< Cl−< Br <I−
2- Charge on the anion :
Significance of the concept of polarization: Fajan Rule
1- Degree of covalent character in an ionic compound:
2- Tendency of a cation to form complexes :
3- Tendency of a cation towards solvation :
4- Nature of oxides:
5- Thermal stability of carbonates:
BeCO3 < MgCO3 < CaCO3 < CaCO3 < SrCO3 < BaCO3
Carbonates : BeCO3 < MgCO3 < CaCO3 < SrCO3 < BaCO3 Decomposition temperature ∘C : 100 < 350 < 547 < 778 < 998
6- Melting points of compounds :
7- Nature of anhydrous halides :
8- Diagonal relationship:
Frequently Asked Questions
What is Fajans Rule?
Fajan Rule are :
The first rule talks approximately about the polarising energy/source of the (+)cation. If the cation could be very small/little, then we will say that the quantity of the ion is much less. If the quantity is much less, we will finish that the rate density of the ion could be high.
Since the rate density is high, the polarising strength of the ion could be high. This makes the compound to be extra covalent.
The 2nd rule talks approximately about the polarizability of the anion. The larger the anion, much less the powerful nuclear rate that holds the valence electron of the ion in place. Since the closing electron is loosely certain in big anions, it could effortlessly be polarised via way of means of a cation, thereby making the compound extra covalent.
The 0.33 rule is a unique case. Let us use an instance to give an explanation for this point.
Example: If we need to discover the extra covalent compound between HgCl2 and Calcium Chloride we can’t use length as a thing to finish. This is due to the fact each Hg2+ and Ca2+ are of just about identical length. To give an explanation for this, we rent the 0.33 rule.
According to fajan’s rule covalent bond is favoured by?
The fajan’s rule covalent bond is favoured by :
- Small cation and Large cation.
- Huge Charge on cation and Small charge on the cation.