Which of the following electronic configuration has highest negative electron gain enthalpy ?

Options

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

Answer

To determine which electronic configuration has the highest negative electron gain enthalpy, we need to analyze the given configurations and understand the concept of electron gain enthalpy.

### Step-by-Step Solution:

1. **Understanding Electron Gain Enthalpy**:
- Electron gain enthalpy is defined as the amount of energy released when an electron is added to an isolated gaseous atom to form a negative ion. A more negative value indicates a greater tendency to gain an electron.

2. **Identifying the Given Configurations**:
- The configurations provided are:
1. \(1s^2 2s^2 2p^6 3s^2 3p^5\) (Chlorine, Cl)
2. \(1s^2 2s^2 2p^5\) (Fluorine, F)
3. \(1s^2 2s^2 2p^6\) (Neon, Ne)
4. \(1s^2 2s^2 2p^6 3s^2\) (Magnesium, Mg)

3. **Analyzing Each Configuration**:
- **Neon (Ne)**:
- Configuration: \(1s^2 2s^2 2p^6\)
- Neon has a complete octet and is stable. It does not tend to gain an electron, so its electron gain enthalpy is not negative.

- **Magnesium (Mg)**:
- Configuration: \(1s^2 2s^2 2p^6 3s^2\)
- Magnesium also has a complete outer shell (2 electrons in 3s). It does not accept electrons, so its electron gain enthalpy is not negative.

- **Fluorine (F)**:
- Configuration: \(1s^2 2s^2 2p^5\)
- Fluorine has 7 electrons in its outer shell and needs one more to complete its octet. It has a strong tendency to gain an electron, resulting in a significant release of energy (negative electron gain enthalpy).

- **Chlorine (Cl)**:
- Configuration: \(1s^2 2s^2 2p^6 3s^2 3p^5\)
- Chlorine also has 7 electrons in its outer shell and needs one more to complete its octet. However, being larger than fluorine, the added electron experiences less attraction due to increased distance from the nucleus, leading to a less negative electron gain enthalpy compared to fluorine.

4. **Conclusion**:
- Among the configurations analyzed, Fluorine (\(1s^2 2s^2 2p^5\)) has the highest negative electron gain enthalpy because it has a strong tendency to gain an electron and complete its octet, resulting in a significant release of energy.

### Final Answer:
The electronic configuration with the highest negative electron gain enthalpy is \(1s^2 2s^2 2p^5\) (Fluorine).