Question

According to Fajan’s rule, ionic character increases for:
1.Large cation and small anion
2.Small cation and small charge on cation
3.Small cation and large charge on cation
4.Large cation and no charge on cation

Answer 1

Hint:
Polarizing power of cation and anion are the two factors that govern the Fajan’s rule. Polarizing power of cation is directly proportional to the density of the charge. The higher the charge density, the greater the cation's polarising strength, and vice versa.

Complete step by step answer:
The law of Fajan predicts whether a chemical bond would be ionic or covalent.
Fajan's Law Postulates
It is possible to state the rule on the basis of three variables, which are:
Size of ion: Large cation, small anion has high ionic character and small cation and large anion have high covalent character.
Cation charge: The higher the cation charges higher the covalent character of the ionic bond.
Electronic configuration: A covalent character of cation is less which have electronic configuration of (noble gases) than the cation with same charge and size and electronic configuration is which are transition elements.
Therefore, From the “size of ion “postulate - Large cation and small anion have high ionic character.
Hence, the correct answer is (1).

Additional information:
When the cation's polarising capacity and the anion's polarizability is small, it gives more ionic character to the bond.
More covalent character to the bond is provided by smaller cation with large anion, having large positive and negative charge, respectively.

Note:Let’s take two examples and explain the ionic character and covalent characters according to the Fajan’s rules;
For Aluminium Iodide ($Al{I}_3$)
This is an ionic bond created by the transmission of electrons. There is a less effective nuclear charge for the iodine because of greater size. The bonding electrons are therefore less drawn towards the nucleus of iodine.
On the opposite, the three positive charges of aluminium which will pull the  pair of electrons towards itself.
This leads to inadequate separation of the charge for it to be ionic, resulting in covalent character growth.
Aluminium Fluoride ($Al{F}_3$)
This is an ionic bond that has also been created by electron transfer.
But here, the smaller fluorine draws the shared electron pair more towards itself, so there is sufficient separation of charge to make it ionic.

Ionic Characteristic	Covalent Characteristic
Large Cation	Small Cation
Small Anion	Large Anion
Small-charge	Large Charge