Which of the following electronic configuration is not possible according to Hund's rule?

Options

A. `1s^(2) 2s^(2)`
B. `1s^(2) 2s^(1)`
C. `1s^(2) 2s^(2) 2p_(x)^(1) 2p_(y)^(1) 2p_(z)^(1)`
D. `1s^(2) 2s^(2) 2p_(x)^(2)`

Explanation

To determine which electronic configuration is not possible according to Hund's rule, we need to understand the rule itself. Hund's rule states that every orbital in a subshell is singly occupied before any orbital is doubly occupied. This means that when filling orbitals of the same energy (like the p orbitals), we should first place one electron in each orbital before pairing them up.

### Step-by-Step Solution:

1. **Understand Hund's Rule**:
- According to Hund's rule, electrons will fill degenerate orbitals (orbitals of the same energy) singly before any pairing occurs. For example, in the p subshell, we have three orbitals (px, py, pz). We should fill each of these orbitals with one electron before we start pairing them.

2. **Analyze the Given Configurations**:
- Let's look at the provided electronic configurations one by one to see if they adhere to Hund's rule.

3. **Configuration 1: 1s² 2s²**
- This configuration indicates that both the 1s and 2s orbitals are fully filled. This is possible and adheres to Hund's rule.

4. **Configuration 2: 1s² 2s¹ 2p¹**
- Here, the 1s orbital is fully filled, the 2s orbital has one electron, and the 2p orbital has one electron. This is also possible and adheres to Hund's rule.

5. **Configuration 3: 1s² 2s² 2p³**
- In this configuration, the 1s and 2s orbitals are fully filled, and the 2p orbital has three electrons. According to Hund's rule, the three electrons will occupy the three p orbitals (px, py, pz) singly before pairing occurs. This configuration is possible.

6. **Configuration 4: 1s² 2s² 2p² (with both electrons in px)**
- This configuration indicates that both electrons in the 2p subshell are occupying the same orbital (px). According to Hund's rule, this is not allowed because we should first fill each of the three p orbitals singly before pairing. Therefore, this configuration is not possible.

### Conclusion:
The electronic configuration that is not possible according to Hund's rule is **1s² 2s² 2p² (with both electrons in px)**.