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 electron gain enthalpy and which has the least negative, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Electron Gain Enthalpy**:
- Electron gain enthalpy is the amount of energy released when an electron is added to a gaseous atom. A more negative value indicates that more energy is released, meaning it is easier for the atom to gain an electron.

2. **Position of Elements in the Periodic Table**:
- Oxygen (O) and Sulfur (S) are in Group 16 (Chalcogens), while Fluorine (F) and Chlorine (Cl) are in Group 17 (Halogens).
- Oxygen is in the second period, and Sulfur is in the third period. Fluorine is in the second period, and Chlorine is in the third period.

3. **Trends in Electron Gain Enthalpy**:
- As we move across a period from left to right, the electron gain enthalpy becomes more negative due to the increasing nuclear charge, which attracts the added electron more strongly.
- However, there are exceptions due to electronic repulsion, particularly in small atoms like Fluorine.

4. **Comparing the Given Elements**:
- The electron gain enthalpy values are as follows:
- O: -141 kJ/mol
- S: -200 kJ/mol
- F: -328 kJ/mol
- Cl: -349 kJ/mol
- From these values, we can see that Chlorine has the most negative electron gain enthalpy, while Oxygen has the least negative.

5. **Conclusion**:
- Therefore, the element with the most negative electron gain enthalpy is **Chlorine (Cl)**, and the element with the least negative electron gain enthalpy is **Oxygen (O)**.

### Final Answer:
- Most negative electron gain enthalpy: **Chlorine (Cl)**
- Least negative electron gain enthalpy: **Oxygen (O)**