Assertion: `Cu^(2+)`(aq) is less than `Cu^(3+)`(aq) but `Fe^(3+)` is more stable than `Fe^(2+)`(aq)
Reason : Half filled and completely filled subshells are more stable

Options

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

Answer

To analyze the assertion and reason provided in the question, we can break it down step by step:

### Step 1: Understanding the Assertion
The assertion states that `Cu^(2+)` (aq) is less stable than `Cu^(3+)` (aq), while `Fe^(3+)` is more stable than `Fe^(2+)` (aq).

- **Copper (Cu)**:
- The electronic configuration of copper is `[Ar] 3d^{10} 4s^1`.
- When copper loses two electrons to form `Cu^(2+)`, it loses one electron from the 4s orbital and one from the 3d orbital, resulting in `[Ar] 3d^9`.
- For `Cu^(3+)`, it loses an additional electron from the 3d orbital, resulting in `[Ar] 3d^8`.
- The stability of `Cu^(3+)` can be attributed to the fact that it is less prone to oxidation compared to `Cu^(2+)`.

- **Iron (Fe)**:
- The electronic configuration of iron is `[Ar] 3d^6 4s^2`.
- When iron loses two electrons to form `Fe^(2+)`, it results in `[Ar] 3d^6`.
- For `Fe^(3+)`, it loses another electron from the 3d orbital, resulting in `[Ar] 3d^5`.
- The `Fe^(3+)` state is more stable due to the half-filled 3d subshell, which is a stable configuration.

### Step 2: Understanding the Reason
The reason provided states that half-filled and completely filled subshells are more stable.

- This is a well-known principle in chemistry. Half-filled (like `3d^5`) and fully filled (like `3d^{10}`) subshells exhibit extra stability due to symmetry and exchange energy.

### Step 3: Evaluating the Assertion and Reason
Now we evaluate the relationship between the assertion and the reason:

- The assertion is **true**: `Cu^(2+)` is indeed less stable than `Cu^(3+)`, and `Fe^(3+)` is more stable than `Fe^(2+)`.
- The reason is also **true**: Half-filled and completely filled subshells are indeed more stable.

However, the reason does not directly explain the assertion regarding the stability of `Cu^(2+)` and `Cu^(3+)`. The stability of these oxidation states is more influenced by their electrode potentials rather than just the electronic configurations.

### Conclusion
Based on the analysis:
- Both the assertion and reason are true, but the reason does not correctly explain the assertion.

Thus, the correct answer is:
**A. If both assertion and reason are true and the reason is not the correct explanation of assertion.**