Which of the following options does not represent ground state electronic configuration of an atom?

Options

A. `1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)3d^(8)4s^(2)`
B. `1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)3d^(9)4s^(2)`
C. `1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)3d^(10)4s^(1)`
D. `1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)3d^(5)4s^(1)`

Explanation

To determine which option does not represent the ground state electronic configuration of an atom, we will analyze each provided option step by step.

### Step 1: Analyze Option 1
- **Configuration**: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ 4s²
- **Explanation**: The order of filling orbitals follows the Aufbau principle, which states that orbitals are filled from lower to higher energy levels. The 4s orbital is filled before the 3d orbital. Therefore, the correct configuration should be 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁸.
- **Conclusion**: This option is correct.

### Step 2: Analyze Option 2
- **Configuration**: 3d⁹ 4s²
- **Explanation**: The 3d orbital should be fully filled (3d¹⁰) for stability reasons before the 4s orbital is considered. Having 3d⁹ indicates that one electron is missing from the 3d orbital, making it less stable.
- **Conclusion**: This option is incorrect.

### Step 3: Analyze Option 3
- **Configuration**: 3d¹⁰ 4s¹
- **Explanation**: This configuration shows that the 3d orbital is fully filled (3d¹⁰), and the last electron is placed in the 4s orbital. This is a valid configuration for copper.
- **Conclusion**: This option is correct.

### Step 4: Analyze Option 4
- **Configuration**: 3d⁵ 4s²
- **Explanation**: The 3d orbital has 5 electrons, which indicates a half-filled configuration. Half-filled d orbitals are stable, and this configuration is valid for chromium.
- **Conclusion**: This option is correct.

### Final Conclusion
The option that does not represent the ground state electronic configuration of an atom is **Option 2 (3d⁹ 4s²)**.

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