What is Fajan's rule?Explain and their applications
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In inorganic chemistry, Fajans' rules, formulated by Kazimierz Fajans in 1923, are used to predict whether a chemical bond will be covalent or ionic, and depend on the charge on the cation and the relative sizes of the cation and anion.
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Fajans' rules
In inorganic chemistry, Fajans' rules, formulated by Kazimierz Fajans in 1923,[1][2][3]are used to predict whether a chemical bondwill be covalent or ionic, and depend on the charge on the cation and the relative sizes of the cation and anion. They can be summarized in the following table:

Chart illustrating the relationship between atomic and ionic radius
IonicCovalentLow positive chargeHigh positive chargeLarge cationSmall cationSmall anionLarge anion
Thus sodium chloride (with a low positive charge (+1), a fairly large cation (~1 Å) and relatively small anion (0.2 Å) is ionic; but aluminium iodide (AlI3) (with a high positive charge (+3) and a large anion) is covalent.
Polarization will be increased by:
high charge and small size of the cationIonic potential Å Z+/r+ (= polarizing power)High charge and large size of the anionThe polarizability of an anion is related to the deformability of its electron cloud (i.e. its "softness")An incomplete valence shell electron configurationNoble gas configuration of the cation produces better shielding and less polarizing powere.g. Hg2+ (r+ = 102 pm) is more polarizing than Ca2+ (r+ = 100 pm)
The "size" of the charge in an ionic bond depends on the number of electrons transferred. An aluminium atom, for example, with a +3 charge has a relatively large positive charge. That positive charge then exerts an attractive force on the electron cloud of the other ion, which has accepted the electrons from the aluminium (or other) positive ion.
In inorganic chemistry, Fajans' rules, formulated by Kazimierz Fajans in 1923,[1][2][3]are used to predict whether a chemical bondwill be covalent or ionic, and depend on the charge on the cation and the relative sizes of the cation and anion. They can be summarized in the following table:

Chart illustrating the relationship between atomic and ionic radius
IonicCovalentLow positive chargeHigh positive chargeLarge cationSmall cationSmall anionLarge anion
Thus sodium chloride (with a low positive charge (+1), a fairly large cation (~1 Å) and relatively small anion (0.2 Å) is ionic; but aluminium iodide (AlI3) (with a high positive charge (+3) and a large anion) is covalent.
Polarization will be increased by:
high charge and small size of the cationIonic potential Å Z+/r+ (= polarizing power)High charge and large size of the anionThe polarizability of an anion is related to the deformability of its electron cloud (i.e. its "softness")An incomplete valence shell electron configurationNoble gas configuration of the cation produces better shielding and less polarizing powere.g. Hg2+ (r+ = 102 pm) is more polarizing than Ca2+ (r+ = 100 pm)
The "size" of the charge in an ionic bond depends on the number of electrons transferred. An aluminium atom, for example, with a +3 charge has a relatively large positive charge. That positive charge then exerts an attractive force on the electron cloud of the other ion, which has accepted the electrons from the aluminium (or other) positive ion.
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