Which of the following has largest negative electron gain enthalpy ?

Options

A. F
B. Cl
C. Br
D. I

Answer

To determine which of the following elements has the largest negative electron gain enthalpy, we need to analyze the electron gain enthalpy trend in the periodic table, particularly for the halogens: fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

### Step-by-Step Solution:

1. **Understanding Electron Gain Enthalpy**:
- Electron gain enthalpy (ΔH) is the energy change when an atom in the gas phase gains an electron. A more negative ΔH indicates a greater tendency to accept an electron, meaning the process is more exothermic.

2. **Periodic Trend**:
- Generally, electron gain enthalpy becomes more negative as you move up a group in the periodic table. This is because smaller atoms have a stronger effective nuclear charge that can attract additional electrons more effectively.

3. **Analyzing Halogens**:
- The halogens are in Group 17 of the periodic table:
- Fluorine (F)
- Chlorine (Cl)
- Bromine (Br)
- Iodine (I)

4. **Expected Trend**:
- Following the general trend, we would expect fluorine to have the most negative electron gain enthalpy because it is the smallest atom in the group.

5. **Exception to the Trend**:
- However, there is an important exception: while fluorine is indeed small, it has significant inter-electronic repulsion due to its high electronegativity and the presence of electrons in the same shell. This repulsion makes it less stable when gaining an additional electron compared to chlorine.

6. **Conclusion**:
- Chlorine (Cl) has the largest negative electron gain enthalpy among the halogens. Although fluorine is smaller, the inter-electronic repulsion in fluorine makes it less favorable for gaining an electron compared to chlorine.

### Final Answer:
- **Chlorine (Cl)** has the largest negative electron gain enthalpy.