Why is the electron gain enthalpy of O or F less than that of S or Cl?

Options

A. O and F are more electronegative than S and Cl.
B. When an electron is added to O or F,it goe to a smaller(n=2) level and suffers more repulsion than the electron in S or Cl in larger level(n=3)
C. Adding an electron to 3p-orbital leads to more rpulsion than 2p-orbital.
D. Electron gain enthalpy depends upon the electron affinity of the atom.

Explanation

To understand why the electron gain enthalpy of oxygen (O) or fluorine (F) is less than that of sulfur (S) or chlorine (Cl), we need to analyze the factors affecting electron gain enthalpy.

### Step-by-Step Solution:

1. **Definition of Electron Gain Enthalpy**:
- Electron gain enthalpy is the amount of energy released when an electron is added to a neutral atom in the gaseous state to form a negative ion.

2. **Factors Influencing Electron Gain Enthalpy**:
- Two primary factors influence electron gain enthalpy:
- **Atomic Radius**: The distance between the nucleus and the outermost electrons.
- **Nuclear Charge**: The total charge of the nucleus due to protons, which affects the attraction between the nucleus and the added electron.

3. **Comparison of Atomic Radius**:
- Oxygen and fluorine have a smaller atomic radius compared to sulfur and chlorine.
- For example:
- Oxygen: 2,6 (2 shells)
- Fluorine: 2,7 (2 shells)
- Sulfur: 2,8,6 (3 shells)
- Chlorine: 2,8,7 (3 shells)
- The smaller atomic radius in O and F means that the added electron is closer to the nucleus.

4. **Electron-Electron Repulsion**:
- When an electron is added to an atom, it experiences repulsion from the electrons already present.
- In the case of O and F, the added electron goes into a smaller energy level (2p), where it experiences greater electron-electron repulsion due to being closer to the nucleus and the other electrons.
- In contrast, for S and Cl, the added electron goes into a larger energy level (3p), where the repulsion is less intense because the electrons are further apart.

5. **Conclusion**:
- Due to the smaller atomic radius and greater electron-electron repulsion in O and F, their electron gain enthalpy is less than that of S and Cl. Thus, when an electron is added to O or F, the energy released is lower compared to the energy released when an electron is added to S or Cl.

### Final Answer:
The electron gain enthalpy of oxygen (O) or fluorine (F) is less than that of sulfur (S) or chlorine (Cl) primarily because when an electron is added to O or F, it goes to a smaller energy level (2p) and suffers more repulsion from existing electrons than the electron added to the larger energy level (3p) in S or Cl.

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