Question

Which of the following is the strongest bond?
(A) $Na-Cl$
(B) $Cs-Cl$
(C) Both (A) and (B)
(D) None of these

Answer 1

Strength of an ionic bond is directly proportional to the covalent character. The percentage of covalent character can be determined using Fajan's rule. An important factor in this rule is the size of the cation. So, compare the size of sodium and cesium ions to apply Fajan's rule as the anion is common to both.

Complete step by step answer:
We classify compounds as ionic and covalent. However, when we have to determine which compound is most ionic among a 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 cations. 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 consider the first rule because the anion is common to both the compounds. When we compare the ionic radius of caesium and sodium, we come to know that the ionic radius of caesium is lesser than sodium.
From this we can conclude that $Cs-Cl$ possessed a stronger bond than $Na-Cl$ as it has greater covalent character.
So, the correct answer is “Option B”.

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.