Which of the following configurations does not follow Hund's rule of maximum multiplicity?

Options

A. `1s^(2)2s^(2)2p^(6)3s^(2)3p^(2)`
B. `1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(2)3d^(6)`
C. `1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(1)3d^(5)`
D. `1s^(2)2s^(2)2p^(6)3s^(2)3p^(4)4s^(2)`

Explanation

To determine which electronic configuration does not follow Hund's rule of maximum multiplicity, we need to understand the rule itself. Hund's rule states that:

1. Every orbital in a given subshell is singly occupied with one electron before any orbital is doubly occupied.
2. All singly occupied orbitals have electrons with the same spin.
3. Only after all orbitals are singly occupied do we start pairing electrons in the orbitals.

Now, let's analyze each option provided in the question:

### Step 1: Analyze the configurations

1. **Option A: 1s² 2s² 2p⁶ 3s² 3p⁶**
- This configuration is completely filled. All orbitals are occupied according to Hund's rule. **(Follows Hund's rule)**

2. **Option B: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶**
- Here, the 4s orbital is filled before the 3d orbitals. This configuration is also correct as it follows Hund's rule. **(Follows Hund's rule)**

3. **Option C: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵**
- This configuration shows that one electron from the 4s orbital has moved to the 3d orbital to achieve a half-filled state in the 3d subshell. This is an exception to Hund's rule but is still considered stable. **(Follows Hund's rule)**

4. **Option D: 1s² 2s² 2p⁶ 3s² 3p⁴ 4s²**
- In this configuration, there are two electrons in the 4s orbital before the 3p orbital is completely filled (which should have 6 electrons). According to Hund's rule, the 3p orbitals should be filled first before moving to the 4s orbital. **(Does not follow Hund's rule)**

### Conclusion
The configuration that does not follow Hund's rule of maximum multiplicity is **Option D**.

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