All noble gases have positive electron gain enthalpy where trend in decreasing order is:
A. \(He\)
B. \(Ne\)
C. \(Ar\)
D. \(Kr\)
E. \(Xe\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(Ne > Ar \approx Kr > Xe > He\)
Noble gases have stable, completely filled electronic configurations (ns2np6, or 1s2 for He). An added electron would have to enter the next shell, so they have no tendency to accept an electron and their electron gain enthalpies are large and positive (energy has to be supplied).
Placing each gas by its value (kJ mol−1), highest first:
B. Ne — first, 116. The added electron would enter the new 3s shell, close to the nucleus, where it is repelled most.
C. Ar and D. Kr — next, about equal at 96 each.
E. Xe — fourth, 77. The new shell lies still farther from the nucleus, so less energy is needed.
A. He — last, 48. It is the exception at the top of the group and has the lowest value of all.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Electron Gain Enthalpy – “Since noble gases have stable electronic configurations, they have no tendency to accept the electron and therefore, have large positive values of electron gain enthalpy.”
Atomic and Physical Properties of Group 18 Elements (row of Table 7.12)
| Property | He | Ne | Ar | Kr | Xe | Rn |
|---|---|---|---|---|---|---|
| Electron gain enthalpy/kJ mol−1 | 48 | 116 | 96 | 96 | 77 | 68 |