what is fajans rule
Answers
Fajans’ rule predicts whether a chemical bond will be covalent or ionic. Few ionic bonds have partial covalent characteristics which were first discussed by Kazimierz Fajans in 1923. In the time with the help of X-ray crystallography, he was able to predict ionic or covalent bonding with the attributes like ionic and atomic radius.
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The ability of a cation to polarise an anion is called its polarising ability and the tendency of an anion to get polarised is called its polarisability. The extent of polarisation in an ionic compound is given by the Fajans rules
(i) To show greater covalent character, both the cation and anion should have high charge on them. Higher the positive charge on the cation, greater will be the attraction on the electron cloud of the anion. Similarly higher the magnitude of negative charge on the anion, greater is its polarisability. Hence, the increase in charge on cation or in anion increases the covalent character. Let us consider three ionic compounds aluminum chloride, magnesium chloride and sodium chloride. Since the charge of the cation increase in the order Na+ < Mg²+ <Al³+ ,the covalent character also follows the same order NaCl <MgCl2 <AICI3.
(ii) The smaller cation and larger anion show greater covalent character due to the greater extent of polarisation. Lithium chloride is more covalent than sodium chloride, e size of Li+ is smaller than Na+ and hence the polarising power of Li+ is more. Lithium iodide is more covalent than lithium chloride as the size of I - is larger than the CI-. Hence I- will be more polarised than Cl- by the cation, Li+
(iii) Cations having ns² np⁶ nd¹⁰ configuration show greater polarising power than the cations with ns² np⁶ configuration. Hence, they show greater covalent character.