Question

In which of the following compounds is the highest covalent character found?

Answer 1

Try to recall Fajan's rule given in the chapter chemical bonding. Fajan's rule is used to determine the covalent character present in an ionic compound. It says that the covalent character of an ionic compound increases when the size of negative ions increases.

Complete answer:
We classify compounds as ionic and covalent. However, when we have to determine which compound is the most ionic among few ionic compounds, we use the rules given by Fajan to answer the question.
Fajan's Rules:
First Rule :
- It speaks about the polarising power of the action. If the cation is smaller, then we can say that the volume occupied by the ion is less. If the volume is less, we can conclude that the charge density is relatively high.
- Due to high charge density, we consider that the polarising power of the ion would be high. This increases the covalent character of the compound.
Second Rule:
- The second rule pertains to the polarizability of the anion. If the size of the ion is large, the effective nuclear charge of that particular electron in the valence shell is less. Since the valence electron is loosely bound in large anions, it can easily be polarised by a cation hence increasing the covalent character of the compound.
Based on these two rules we will answer the question mentioned above.
We will not consider rule 1, as the cation is common to all the 4 compounds ($C{{a}^{+}}$). Applying the second rule we get to know that Iodide ion(${{I}^{{}}}^{-}$) size is the largest thereby increasing the covalent character of the compound. The compound with the highest covalent character is $Ca{{I}_{2}}$.

Therefore, the correct answer is option (D).

Note:
Fajan gave a third rule which is applied only under certain circumstances. The rule is applied when the anion is the same and the size of two different cations are relatively the same.
The rule says that for cations with same charge and size, the cation with electronic configuration $(\text{n-1)}{{\text{d}}^{\text{n}}}\text{n}{{\text{s}}^{0}}$ which is found in transition elements have greater covalent character than the cation with electronic configuration $\text{n}{{\text{s}}^{2}}\text{n}{{\text{p}}^{6}}$ which is found in alkali or alkali earth metals.