The variation of electron gain enthalpy values of halogens are in the order ________.

Answer

To determine the variation of electron gain enthalpy values of halogens, we need to analyze the halogens present in Group 17 of the periodic table: fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

### Step-by-Step Solution:

1. **Identify the Halogens**: The halogens in Group 17 are fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

2. **Understand Electron Gain Enthalpy**: Electron gain enthalpy is the energy change that occurs when an electron is added to a neutral atom in the gas phase. A more negative value indicates a greater tendency to gain an electron.

3. **Consider Atomic Size**: As we move down the group from fluorine to iodine, the atomic size increases. Larger atoms have their valence electrons further from the nucleus, which generally leads to a decrease in electron gain enthalpy.

4. **Analyze Fluorine's Unique Case**: Although fluorine is the smallest atom in the group, it has a high electron density due to its small size. This leads to increased inter-electronic repulsion among its valence electrons, which can affect its ability to gain an additional electron.

5. **Compare Electron Gain Enthalpy Values**:
- Chlorine (Cl): -349 kJ/mol
- Fluorine (F): -328 kJ/mol
- Bromine (Br): -325 kJ/mol
- Iodine (I): -295 kJ/mol

6. **Establish the Order**: Based on the values and the factors discussed:
- Chlorine has the highest electron gain enthalpy (most negative).
- Fluorine follows, despite its small size.
- Bromine comes next.
- Iodine has the least negative value.

7. **Final Order**: Therefore, the variation of electron gain enthalpy values of halogens is in the order:
**Cl > F > Br > I**