To determine the correct order of electron affinity for the elements fluorine (F), chlorine (Cl), sulfur (S), and oxygen (O), we need to consider several factors, including the size of the atoms, their electronegativity, and the presence of lone pairs of electrons.
### Step-by-Step Solution:
1. **Understanding Electron Affinity**:
- Electron affinity is the energy change that occurs when an electron is added to a neutral atom in the gas phase. A higher electron affinity means that the atom has a greater tendency to gain an electron.
2. **Group and Period Considerations**:
- Fluorine and chlorine are halogens (Group 17), while oxygen and sulfur are chalcogens (Group 16).
- Fluorine and oxygen are in the second period, while chlorine and sulfur are in the third period.
3. **Electronegativity Trends**:
- Electronegativity generally increases across a period (left to right) and decreases down a group (top to bottom).
- Fluorine has the highest electronegativity, followed by chlorine, oxygen, and then sulfur.
4. **Size of Atoms**:
- As we move down a group, the size of the atoms increases. Chlorine is larger than fluorine, and sulfur is larger than oxygen.
- Smaller atoms tend to have higher electron affinity due to their ability to attract electrons more effectively.
5. **Lone Pair Repulsion**:
- Both fluorine and oxygen have lone pairs of electrons, which can repel incoming electrons, reducing their electron affinity.
- Fluorine, being smaller, has a stronger repulsion effect due to its lone pairs compared to chlorine.
6. **Comparing Electron Affinities**:
- **Fluorine vs. Chlorine**: Despite fluorine's higher electronegativity, its small size and lone pair repulsion result in a lower electron affinity compared to chlorine.
- **Oxygen vs. Sulfur**: Oxygen's smaller size leads to greater repulsion from its lone pairs, making its electron affinity lower than that of sulfur.
7. **Final Order**:
- Based on the above considerations, the order of electron affinity from highest to lowest is:
- Cl > F > S > O
### Conclusion:
The correct order of electron affinity for the elements is:
**Cl > F > S > O**
