In an ionic compound `A^(+) X^(-)` the degree of covalent bonding is greatest when

Options

A. `A^(+) and X^(-)` ion are small
B. `A^(+)` is small and `X^(-)` is large
C. `A^(+) and X^(-)` ions are approximately of the same size
D. `X^(-)` is small and `A^(+)` is large

Answer

To determine when the degree of covalent bonding is greatest in the ionic compound \( A^{+}X^{-} \), we can analyze the factors affecting covalent character in ionic compounds based on Fajans' rules.

### Step-by-Step Solution:

1. **Understanding Ionic Compounds**:
- An ionic compound consists of cations (positively charged ions) and anions (negatively charged ions). In this case, \( A^{+} \) is the cation and \( X^{-} \) is the anion.

2. **Fajans' Rules**:
- Fajans' rules help us understand the conditions under which ionic compounds exhibit covalent character. The rules state that covalent character increases when:
- The cation is small and has a high charge.
- The anion is large and has a high charge.

3. **Analyzing the Options**:
- **Option 1**: Cation and anion are small.
- A small cation has high polarizing power, but a small anion has low polarizability. This combination does not maximize covalent character.
- **Option 2**: Cation is small and anion is large.
- A small cation will have high polarizing power, and a large anion will have high polarizability. This scenario leads to a greater distortion of the electron cloud of the anion, enhancing covalent character.
- **Option 3**: Cation and anion are approximately the same size.
- This may lead to some covalent character, but it is not optimal compared to the previous option.
- **Option 4**: Anion is small and cation is large.
- A large cation has low polarizing power, and a small anion has low polarizability. This combination minimizes covalent character.

4. **Conclusion**:
- The degree of covalent bonding in the ionic compound \( A^{+}X^{-} \) is greatest when the cation \( A^{+} \) is small and the anion \( X^{-} \) is large. Therefore, the correct answer is **Option 2**.

### Final Answer:
The degree of covalent bonding is greatest when the cation is small and the anion is large.

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