Question

Which of the following compounds has the lowest melting point?
(A) $CaC{l_2}$
(B) $CaB{r_2}$
(C) $Ca{I_2}$
(D) $Ca{F_2}$

Answer 1

We very well know that alkali earth metals are smaller in size and their melting points and boiling points are higher than that of the alkali metals. And we know that according to Fajan’s rule, the element with maximum covalent character will have a low melting point.

Complete answer: As we know that Fajan’s rule states that anion having the greater size will get easily polarised and thus will have a minimum covalent character and a cation having smaller size will be having higher covalent characters.
We also know that if the covalent character is maximum, then the melting point will be the lowest for the particular compound because the ionisation enthalpy is low and as the covalent character increases, ionic character of that compound decreases.
So, from the above explanation if we see our given chemical species, the cation will be calcium as $C{a^{2 + }}$and the cation is similar in all the given options but when we talk about the anions, they belong to halogen groups which are arranged accordingly as shown below:
$F > Cl > Br > I$.
And we know that while moving down the group the size increases which results in maximum size of the element iodine and thus the covalent character will be highest or maximum in the iodide ion and hence the melting point will also be lowest.
Thus we can say that calcium iodide $(Ca{I_2})$ will have the lowest melting point among the given options.

Hence the correct answer is (C).

Note: Always remember that while moving down the group of the periodic table the size of the element always increases because the number of shells keeps on adding while moving down and thus the ionisation energy decreases down the group resulting in maximum covalent character and minimum ionic character.