JEE Main 2023 · 25 January · morning · Single correct · +4 / −1
Inert gases have positive electron gain enthalpy. Its correct order is :
- (a) He < Ne < Kr < Xe
- (b) He < Xe < Kr < Ne
- (c) Xe < Kr < Ne < He
- (d) He < Kr < Xe < Ne
Answer and solution
Answer (b) He < Xe < Kr < Ne
Solution
Chemistry ma’am · 3 steps
Neon is the most positive and helium the least; between them the values fall as the atoms get bigger.
- Why positive. A noble gas already has a full valence shell. An extra electron must go into the next, higher shell, so energy has to be supplied and Δ_(eg)H is positive.
- Trend from neon. Neon is small, so the new electron in its 3s orbital is crowded by eight tightly held 2s² 2p⁶ electrons. Going down to Ar, Kr and Xe, the atoms grow and this repulsion eases, so the values become less positive. Helium is the exception: it has only two electrons to repel the incoming one, so its value is the smallest.
Gas Δ_(eg)H / kJ mol⁻¹ He +48, least positive Ne +116, most positive Kr +96 Xe +77 - So. In increasing order the values are 48 < 77 < 96 < 116, that is He < Xe < Kr < Ne. This is option (b).
The idea to keep. For noble gases, remember that neon has the highest electron gain enthalpy and helium the lowest. From Ne down to Xe the values drop as the atoms get bigger.
Answer from the NTA final answer key. Solution by PrepWiser.