Question

The set containing the elements with positive electron gain enthalpies is?

• A. S, Se, Te
• B. Kr, Xe, Rn
• C. Cl, Br, I
• D. K, Rb, Cs

Hint:

Noble gases have positive electron gain enthalpy because their full valence shells make adding an extra electron energetically unfavorable.

Answer 1

The Correct Option is B

Step 1: Understanding Electron Gain Enthalpy 
Electron gain enthalpy (\( \Delta H_{eg} \)) is the energy change when an atom gains an electron. It can be:
Negative (exothermic): When energy is released, meaning the atom strongly attracts electrons (e.g., halogens).
Positive (endothermic): When energy must be supplied to add an electron, meaning the atom resists gaining electrons (e.g., noble gases).
Step 2: Identifying Elements with Positive Electron Gain Enthalpy 
- Option (1): S, Se, Te Incorrect. These elements belong to group 16 (oxygen family) and generally have negative electron gain enthalpy.
- Option (2): Kr, Xe, Rn  Correct. These noble gases have completely filled electronic configurations, making electron addition unfavorable, resulting in a positive electron gain enthalpy.
- Option (3): Cl, Br, I Incorrect. These are halogens with high electron affinity and negative electron gain enthalpy.
- Option (4): K, Rb, Cs  Incorrect. These alkali metals have low electron affinity but still have negative electron gain enthalpy.

 Step 3: Verify the Correct Answer 
Since noble gases resist electron gain due to their stable configuration, the set with positive electron gain enthalpy is Kr, Xe, Rn (Option 2).