Which electronic configuration does not follow the Aufbau rule? 

Options

A. `1s^2, 2s^2, 2p^6`
B. `1s^2,2s^2, 2p^4, 3s^2`
C. `1s^2`
D. `1s^2, 2s^2, 2p^6, 3s^3`

Answer

To determine which electronic configuration does not follow the Aufbau principle, we need to analyze the given configurations based on the principle itself. The Aufbau principle states that electrons occupy the lowest energy orbitals first before moving to higher energy orbitals.

### Step-by-Step Solution:

1. **Understand the Aufbau Principle**:
- According to the Aufbau principle, electrons fill orbitals starting from the lowest energy level to the highest. The order of filling is determined by the n + l rule, where 'n' is the principal quantum number and 'l' is the azimuthal quantum number.

2. **Identify the Electronic Configurations**:
- We will analyze the configurations provided:
- Configuration 1: 1s² 2s² 2p⁶ 3s²
- Configuration 2: 1s² 2s² 2p⁶ 3s² 3p¹

3. **Calculate n + l Values**:
- For each subshell, calculate the n + l value:
- For 1s: n = 1, l = 0 → n + l = 1
- For 2s: n = 2, l = 0 → n + l = 2
- For 2p: n = 2, l = 1 → n + l = 3
- For 3s: n = 3, l = 0 → n + l = 3
- For 3p: n = 3, l = 1 → n + l = 4

4. **Analyze the Filling Order**:
- Based on the n + l values, the correct filling order should be:
- 1s (1) → 2s (2) → 2p (3) → 3s (3) → 3p (4)
- This means that 3s should be filled before 3p.

5. **Check Each Configuration**:
- Configuration 1: 1s² 2s² 2p⁶ 3s²
- This follows the Aufbau principle as it fills the orbitals in the correct order.
- Configuration 2: 1s² 2s² 2p⁶ 3s² 3p¹
- This configuration also follows the Aufbau principle as it fills 3s before 3p.

6. **Identify the Incorrect Configuration**:
- If we had a configuration like 1s² 2s² 3s² 2p⁶, this would not follow the Aufbau principle because 3s is filled before 2p, which has a lower energy level.

7. **Conclusion**:
- The electronic configuration that does not follow the Aufbau principle is the one where 3s is filled before 2p.

### Final Answer:
The electronic configuration that does not follow the Aufbau rule is **1s² 2s² 3s² 2p⁶**.