Question
Explain Fajan’s rule with suitable examples.
✓
Answer
Fajan’s rule:
$i.$ Smaller the size of the cation and larger the size of the anion, greater is the covalent character of the ionic bond. For example$, Li^+ Cl^–$ is more covalent than $Na^+Cl.$ Similarly$, Li^+I^–$ is more covalent than $Li^+Cl^–.$
$ii.$ Greater the charge on cation, more is covalent character of the ionic bond. For example, covalent character of $AlCl_3, MgCl_2$ and $NaCl$ decreases in the following order $Al^{3+}(Cl^–)_3 > Mg^{2+}(Cl^–)_2 > Na^+ Cl^–$
$iii.$ A cation with the outer electronic configuration of the $s^2p^6d^{10}$ type possesses greater polarising power compared to the cation having the same size and same charge but having outer electronic configuration of $s^2p^6$ type.
This is because d electrons of the $s^2p^6d^{10}$ shell screen nuclear charge less effectively compared to $s$ and $p$ electrons of $s^2p^6$ shell. Hence, the effective nuclear charge in a cation having $s^2p^6d^{10}$ configuration is greater than that of the one having $s^2p^6$ configuration. For example$: Cu^+Cl^–$ is more covalent than $Na^+Cl^–.$ Here,
$(Cu^+ = 1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10}; Na^+ = 1s^2 2s^2 2p^6)$
$i.$ Smaller the size of the cation and larger the size of the anion, greater is the covalent character of the ionic bond. For example$, Li^+ Cl^–$ is more covalent than $Na^+Cl.$ Similarly$, Li^+I^–$ is more covalent than $Li^+Cl^–.$
$ii.$ Greater the charge on cation, more is covalent character of the ionic bond. For example, covalent character of $AlCl_3, MgCl_2$ and $NaCl$ decreases in the following order $Al^{3+}(Cl^–)_3 > Mg^{2+}(Cl^–)_2 > Na^+ Cl^–$
$iii.$ A cation with the outer electronic configuration of the $s^2p^6d^{10}$ type possesses greater polarising power compared to the cation having the same size and same charge but having outer electronic configuration of $s^2p^6$ type.
This is because d electrons of the $s^2p^6d^{10}$ shell screen nuclear charge less effectively compared to $s$ and $p$ electrons of $s^2p^6$ shell. Hence, the effective nuclear charge in a cation having $s^2p^6d^{10}$ configuration is greater than that of the one having $s^2p^6$ configuration. For example$: Cu^+Cl^–$ is more covalent than $Na^+Cl^–.$ Here,
$(Cu^+ = 1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10}; Na^+ = 1s^2 2s^2 2p^6)$
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