Which of the following will have the most negative electron gain enthalpy and which the least negative? `O, S, F` and `CI`. Explain.

Answer

To determine which of the elements O (Oxygen), S (Sulfur), F (Fluorine), and Cl (Chlorine) has the most negative and least negative electron gain enthalpy, we need to analyze the factors affecting electron gain enthalpy.

### Step-by-Step Solution:

1. **Understanding Electron Gain Enthalpy**:
- Electron gain enthalpy is the energy change when an electron is added to a neutral atom in the gaseous state. A more negative value indicates a greater tendency to gain an electron.

2. **Trends in Electron Gain Enthalpy**:
- **Across a Period**: As we move from left to right across a period, the electron gain enthalpy becomes more negative due to increasing nuclear charge, which attracts the added electron more effectively.
- **Down a Group**: As we move down a group, the size of the atoms increases, and the added electron is farther from the nucleus. This results in a less negative electron gain enthalpy.

3. **Analyzing the Elements**:
- **Fluorine (F)**: Being in Group 17 (halogens), it has a high tendency to gain an electron. However, due to its small size, the added electron experiences significant electron-electron repulsion, making its electron gain enthalpy less negative compared to chlorine.
- **Chlorine (Cl)**: Also in Group 17, but larger than fluorine, chlorine can accommodate an extra electron more easily without as much repulsion, resulting in a more negative electron gain enthalpy than fluorine.
- **Oxygen (O)**: In Group 16, oxygen has a smaller size compared to sulfur, leading to a more negative electron gain enthalpy than sulfur. However, it is less negative than that of chlorine and fluorine due to its small size causing instability with the added electron.
- **Sulfur (S)**: Being larger than oxygen, sulfur has a less negative electron gain enthalpy than oxygen.

4. **Conclusion**:
- **Most Negative Electron Gain Enthalpy**: Chlorine (Cl) has the most negative electron gain enthalpy among the given elements.
- **Least Negative Electron Gain Enthalpy**: Oxygen (O) has the least negative electron gain enthalpy.

### Summary:
- **Most Negative**: Cl (Chlorine)
- **Least Negative**: O (Oxygen)