The correct order of electron gain enthalpy (in kJ/mole) is

Options

A. ` F gt Cl `
B. `O gt S `
C. `S gt O `
D. `I gt Br `

Explanation

To determine the correct order of electron gain enthalpy, we need to analyze the trends in electron gain enthalpy for different groups in the periodic table, particularly focusing on the halogens and the chalcogens (oxygen family).

### 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 neutral gaseous atom. It can be either negative (exothermic) or positive (endothermic). Generally, a more negative value indicates a greater tendency to gain an electron.

2. **Analyzing Halogens**:
- The halogens include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).
- The order of electron gain enthalpy for halogens is generally:
\[ \text{F} > \text{Cl} > \text{Br} > \text{I} \]
- However, fluorine has a less negative electron gain enthalpy compared to chlorine due to the small size of the fluorine atom, which leads to increased electron-electron repulsion in its 2p orbital.

3. **Analyzing Chalcogens (Oxygen Family)**:
- The chalcogens include oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po).
- The order of electron gain enthalpy for chalcogens is generally:
\[ \text{O} > \text{S} > \text{Se} > \text{Te} > \text{Po} \]
- Oxygen has a lower electron gain enthalpy compared to sulfur due to the high electron-electron repulsion in its small 2p orbital.

4. **Comparing Groups**:
- From the analysis, we find that:
- For halogens: \( \text{F} > \text{Cl} > \text{Br} > \text{I} \)
- For chalcogens: \( \text{S} > \text{O} > \text{Se} > \text{Te} > \text{Po} \)

5. **Final Order**:
- Combining both groups, the overall order of electron gain enthalpy is:
\[ \text{F} > \text{Cl} > \text{Br} > \text{I} > \text{S} > \text{O} > \text{Se} > \text{Te} > \text{Po} \]

### Conclusion:
The correct order of electron gain enthalpy is:
\[ \text{F} > \text{Cl} > \text{Br} > \text{I} > \text{S} > \text{O} > \text{Se} > \text{Te} > \text{Po} \]