Question

In which of the following cases the covalent character and the melting point order are the same?
(A) $BeC{{l}_{2}},CaC{{l}_{2}},BaC{{l}_{2}}$
(B) $NaCl,MgC{{l}_{2}},AlC{{l}_{3}}$
(C) $MgB{{r}_{2}},SrB{{r}_{2}},BaB{{r}_{2}}$
(D) $NaF,Mg{{F}_{2}},Al{{F}_{3}}$

Answer 1

Hint: With the help of
 Fajan’s rule we can calculate or find the order of the covalent character of the compounds. Fajan’s rule is based on the size of the cation. The melting point of the compound can be calculated by the force of attraction between the ions.

Complete step by step solution:
Let us understand the covalent character of ionic bonds:
When a cation approaches an anion, the electron cloud of the anion is attracted to the cation and hence gets distorted. The effect is called the polarization of anion. The power of the cation to polarize the anion is called its polarizing power and the tendency of the anion to get polarized is called its polarizability. The greater is the polarization produced, the more is the neutralization of the charge, and hence the ionic character decreases or the covalent character increases. The properties like melting point, the heat of sublimation, and solubility in water also increase.
As the size of the cation decreases, the covalent character increases.
In$NaF, Mg{{F}_{2}}, Al{{F}_{3}}$, the covalent character increases because the size of the cation is decreasing in the order:
$N{{a}^{+}}$ >$M{{g}^{2+}}$ >$A{{l}^{3+}}$ , and also the charge on the cation is also increasing.
Their melting points also increase because as the charge of the cation and anion increases the force of attraction increases hence, the bond strength also increases.

So, the correct answer is an option (d)- $NaF,Mg{{F}_{2}},Al{{F}_{3}}$.

Note: You may get confused that option (b)- $NaCl, MgC{{l}_{2}}, AlC{{l}_{3}}$ is also correct because their covalent character is also in increasing order, but the order of melting point is opposite because sodium chloride is ionic whereas aluminium chloride is a simple molecular compound.