The difference between electron gain enthalpies will be maximum between :

Solution

Chemistry ma’am · 3 steps

For the biggest gap, pair the most positive noble gas with the most negative halogen: Ne with Cl.

1. Two opposite ends. Noble gases have full shells, so an added electron must enter a new, higher shell. Their electron gain enthalpies are positive. Halogens need one electron to complete the octet, so theirs are large and negative.
2. The surprises. Cl is more negative than F, because the small 2p shell of fluorine makes the incoming electron feel strong repulsion. Ne is more positive than Ar, because the new electron would go into the 3s shell of Ne, which is close to the nucleus and well shielded from it by the full shell beneath.

   Pair	Values (kJ mol⁻¹)	Difference (kJ mol⁻¹)
   (a) Ar and Cl	+96 and -349	445
   (b) Ne and Cl	+116 and -349	465
   (c) Ne and F	+116 and -328	444
   (d) Ar and F	+96 and -328	424

3. So. The largest difference, 116 - (-349) = 465 kJ mol⁻¹, is between Ne and Cl. The answer is (b).

The idea to keep. Electron gain enthalpy is most negative for Cl, not F, and most positive for Ne among the noble gases. To maximise a difference, take the extreme on each side.

Answer from the NTA final answer key. Solution by PrepWiser.