(A) : The electronic configuration of Cr is `[Ar]3d^(5)4s^(1)` but not `[Ar]3d^(4)4s^(2)`
(R) : Lowering energy with configuration `[Ar]3d^(5) 4s^(1)` is more than that with configuration `[Ar]3d^(4)4s^(2)`

Options

A. Both (A) and (R) are true and (R) is the correct explanation of (A)
B. Both (A) and (R) are true and (R) is not the correct explanation of (A)
C. (A) is true but (R) is false
D. (A) is false but (R) is true

Answer

To solve the question regarding the electronic configuration of chromium (Cr), we need to analyze both the assertion (A) and the reason (R) provided.

### Step-by-Step Solution:

1. **Understanding Electronic Configuration**:
- The electronic configuration of an element describes the distribution of electrons in its atomic orbitals. For chromium (Cr), the expected configuration based on the Aufbau principle would be `[Ar] 3d^4 4s^2`.

2. **Actual Electronic Configuration of Chromium**:
- However, the actual electronic configuration of chromium is `[Ar] 3d^5 4s^1`. This is an exception to the expected configuration.

3. **Reason for the Exception**:
- The reason for this exception lies in the stability associated with half-filled and fully filled d orbitals. A half-filled d subshell (with 5 electrons) provides extra stability due to:
- **Electron Exchange Energy**: Half-filled and fully filled orbitals allow for greater electron exchange interactions, which lowers the energy of the atom.
- **Symmetry**: A half-filled d subshell has a symmetrical distribution of electrons, which also contributes to stability.

4. **Comparing Energies**:
- The configuration `[Ar] 3d^5 4s^1` is more stable (lower energy) than `[Ar] 3d^4 4s^2` because the half-filled d subshell (3d^5) results in a more stable arrangement than having one less electron in the d subshell and two in the s subshell.

5. **Conclusion**:
- Therefore, the assertion (A) that the electronic configuration of Cr is `[Ar] 3d^5 4s^1` is true, and the reason (R) that this configuration is more stable (lower energy) due to the half-filled d subshell is also true. Thus, both A and R are true, and R correctly explains A.

### Final Answer:
Both the assertion (A) and the reason (R) are true, and R is the correct explanation of A.