Covalent character in ionic bonds
Let’s start with the Covalent character in ionic bonds(Fajan’s Rules)…..children …we know that there are many covalent compounds which show ionic behavior due to polarity, likewise, there are many ionic compounds, which possess partial covalent character and resemble more to covalent compounds in their properties.
LiCl is an ionic compound but it is more soluble in the organic solvent than in water. Cuprous chloride behaves more as a covalent compound than an ionic compound.
Now I am going to explain the facts due to which ionic molecules behave like covalent molecules.
When two oppositely charged ions come close to each other, the positively charged cation attracts the outermost electrons of the anion and repels its nucleus simultaneously.
This causes a distortion of the anion, and its electron density gets drifted toward the cation resulting in some sharing of electrons between the two. This phenomenon is known as POLARISATION. Due to this polarisation, the ionic bond develops a partial covalent character which is shown in the picture below.
Due to polarisation, ionic compounds develop partial covalent character and exhibit many properties characteristic of covalent compounds.
The extent of polarisation in an ionic compound-
The extent of polarisation in an ionic compound is governed by certain rules called Fajan’s rules. These rules are given below:
There are two factors that govern the extent of polarisation. These are-
1-Size of ions
2-Charges on ions
We will discuss all these points one by one.
1-Size of ions-
In an ionic compound, the extent of polarisation is greater if the size of the cation is smaller and the size of the anion is bigger. Why?
The reason is that when a cation is small, the density of positive charges present on its surface is more. Therefore, a small cation can attract the electrons of the anion more efficiently as compared to a larger cation having the same charge.
On the other hand, when an anion is large, its outermost electrons experience lesser pull by its nucleus and more easily distorted.
This is why the extent of polarisation in LiCl (lithium chloride) is more than that in NaCl (sodium chloride) which has a larger cation (sodium ion). Similarly, LiI (lithium iodide) is more covalent than LiCl ( lithium chloride) due to the presence of a larger anion (iodide ion).
2-Charge on ions-
In an ionic compound, the extent of polarisation is greater if the charge of cation and anion is larger.
As the charge on the two ions increases, the electrostatic attraction of the cation for the outer electrons of the anion also increases. This favors the polarisation of the anion.
For example, the covalent character of chlorides of aluminum, magnesium, and sodium follows the following order:
Question– Why is CuCl is more covalent than NaCl?
Answer– CuCl is an ionic compound but shows the covalent character, it depends on the polarisation ability of cation. Smaller cation has a great tendency to polarise the electron of the anion. Cu in CuCl distorts electron clouds more according to fajan’s rule. Therefore, CuCl is more covalent than NaCl.
Question–In each of the following pairs, which is more covalent and why?
(i)- LiCl and LiBr (ii)-LiCl and LiI
Answer (i)- In LiCl and LiBr the cations are the same. But as we compare Cl and Br, the size of Br (anion) is greater than Cl, which distorts electron clouds more according to fajan’s rule. Therefore, LiBr is more covalent than LiCl.
(ii)- In LiCl and LiI the cations are the same but the size of I is greater than Cl, which distorts electron clouds more according to fajan’s rule. Therefore, LiBr is more covalent than LiCl.
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