Given below are two statements: One is labelled as Assertion A and the other labelled as Reason R.
Assertion A : Lithium halides are somewhat covalent in nature.
Reason R : Lithium possess high polarisation capability.
In the light of the above statements, choose the most appropriate answer from the options given below:

We begin by reading the two statements carefully. Assertion A says that lithium halides are somewhat covalent in nature, while Reason R claims that lithium possesses high polarisation capability. In order to judge these statements, we must recall the concept of polarising power and Fajans’ rules.

Fajans’ rules state that the covalent character in an ionic bond increases when:

(i) the cation has a small radius and/or high charge,

(ii) the anion has a large radius and/or high charge,

because under such conditions the cation can strongly distort (polarise) the electron cloud of the anion. This distortion leads to sharing of electrons rather than complete transfer, imparting covalent character to the bond.

Now we apply these rules to lithium halides. The lithium ion is $$\text{Li}^+$$. Among all alkali-metal cations (Li $$^+, \text{Na}^+, \text{K}^+, \ldots)$$, $$\text{Li}^+$$ has the smallest ionic radius. Numerically,

$$r(\text{Li}^+) \approx 0.76\ \text{Å}, \quad r(\text{Na}^+) \approx 1.02\ \text{Å}, \quad r(\text{K}^+) \approx 1.38\ \text{Å}$$

and so on. Because the charge on each of these ions is the same $$(+1)$$, a smaller radius means a larger charge density, i.e.

Charge density $$= \frac{+1}{\text{smaller volume}} \ \Longrightarrow \ \text{larger electric field around Li}^+.$$

The polarising power is directly proportional to this charge density. Therefore $$\text{Li}^+$$ shows the highest polarising power among alkali cations.

Let us see the direct consequence for lithium halides. The general reaction for the formation of a lithium halide is

Li $$^+ + \text{X}^- \longrightarrow \text{LiX}, \quad \text{where } \text{X}^- = \text{F}^-,\ \text{Cl}^-,\ \text{Br}^-,\ \text{I}^-.$$

Because $$\text{Li}^+$$ is so small and highly polarising, it pulls the electron cloud of $$\text{X}^-$$ towards itself. This polarisation produces partial sharing of the electrons between Li and X, and that partial sharing manifests as partial covalent character in the Li-X bond. Thus, lithium halides are indeed somewhat covalent instead of being purely ionic.

Therefore Assertion A is true.

Next we evaluate Reason R. We have just argued—using size, charge density and Fajans’ rules—that $$\text{Li}^+$$ has a high polarising power. Hence Reason R is also true.

Finally, we must determine whether R correctly explains A. Since A (lithium halides are covalent) directly follows from the high polarising capability of $$\text{Li}^+$$ as described in R, Reason R is the correct and relevant explanation of Assertion A.

Hence, the correct answer is Option D.