Question

By Fajan’s rules, covalent character increases with increase in size of the anion. Identify the correct order of melting point of calcium halides.
A) $Ca{ F }_{ 2 }$<$Ca{ Cl }_{ 2 }$<$Ca{ Br }_{ 2 }$<$CaI_{ 2 }$
B) $Ca{ Cl }_{ 2 }$<$Ca{ F }_{ 2 }$<$Ca{ Br }_{ 2 }$<$CaI_{ 2 }$
C) $Ca{ F }_{ 2 }$>$Ca{ Cl }_{ 2 }$>$Ca{ Br }_{ 2 }$>$CaI_{ 2 }$
D) $Ca{ Cl }_{ 2 }$>$Ca{ F }_{ 2 }$>$Ca{ Br }_{ 2 }$>$CaI_{ 2 }$

Answer 1

Hint: In this question, we are comparing the melting point for calcium halides. In all these compounds the cation is the same but the anion is changing from fluoride ion to iodide ion. In such a case we should show the covalent character is changing in each compound since greater the covalent character, less will be the melting point.

Complete step by step solution:
Some compounds have both covalent character as well as ionic character. In some of them the covalent character dominates while in the others the ionic character dominates. In order to predict the covalent character in ionic compounds, Kazimierz Fajans proposed a set of rules in 1923.
Given below are the Fajan’s rules
If a compound has small cation and large anion, the covalent character in the compound will increase. A small cation has a high polarising power. Polarising power depends upon the charge density. Greater the charge density (ratio of charge to volume) greater will be the polarising power of the cation. Similarly larger the anion more will be its polarisability. As the size of the anion increases, the effective nuclear charge on the valence electrons decreases such that they can be easily polarised by the cation.
If the size and charge on the cations are same, then the cations with the electronic configuration (n-1)dnnso i.e. compounds with transition metal cations will have greater covalent character than the alkali or alkaline metal cations. This is because the d orbitals poorly shield the effective nuclear charge due to which the polarising power of the cation will increase.
The melting point of a compound depends on its lattice enthalpy which in turn depends upon the charge on ions and their ionic radii. Greater the lattice energy, greater will be the melting point and lower will be the solubility. $Ca{ F }_{ 2 }$ will have the highest melting point since it has a very high lattice enthalpy. The decreasing order of melting points for other calcium halides is as follows $Ca{ Cl }_{ 2 }$>$Ca{ Br }_{ 2 }$>$CaI_{ 2 }$. This is because the degree of covalent character is increasing as we move from fluorides to iodides with calcium iodide having the most covalent character as can be seen according to Fajan’s rules.
Therefore the correct answer is option (C) $Ca{ F }_{ 2 }$>$Ca{ Cl }_{ 2 }$>$Ca{ Br }_{ 2 }$>$CaI_{ 2 }$.

Note: If the cation in a compound has greater charge along with small ionic radii, the lattice enthalpy will be higher. Similarly if an anion in a compound has more charge and small ionic radii, the lattice enthalpy of the compound will be higher.