Assertion , `d_(5)` configuration is more stable than `d_(4)`
Reason : `d_(5)` has more exchange energy as compared to `d_(4)`
because 10 & 6 exchanges are possible in `d_(5) & d_(4)` respectively.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the problem, we need to analyze the assertion and reason provided regarding the stability of d5 and d4 configurations in terms of exchange energy.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the d5 configuration is more stable than the d4 configuration. This is generally true in transition metals where half-filled and fully filled d-orbitals tend to be more stable due to symmetry and exchange energy.

2. **Understanding the Reason**:
- The reason given is that d5 has more exchange energy compared to d4 because there are 10 and 6 possible exchanges in d5 and d4 respectively.

3. **Calculating Exchange Energy**:
- **For d4 Configuration**:
- In a d4 configuration, we can visualize the arrangement of electrons in the d-orbitals. The possible exchanges can be calculated based on the number of ways electrons can be paired or exchanged.
- The total number of exchanges possible in d4 is found to be 6.

- **For d5 Configuration**:
- In a d5 configuration, the arrangement allows for more exchanges due to the presence of an additional electron.
- The total number of exchanges possible in d5 is calculated to be 10.

4. **Comparing Exchange Energies**:
- Since d5 has more exchange energy (10) compared to d4 (6), it indicates that d5 is more stable due to the greater stabilization from exchange interactions.

5. **Conclusion**:
- Both the assertion and reason are correct. The assertion that d5 is more stable than d4 is supported by the reason that d5 has more exchange energy due to a greater number of possible exchanges.

### Final Answer:
Both the assertion and reason are correct, and the reason correctly explains why d5 is more stable than d4.

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