Q.

Explanation:

Electron gain enthalpy refers to the energy change when an electron is added to a neutral atom in the gaseous state. For noble gases, this process is endothermic, resulting in positive electron gain enthalpies due to their stable, fully filled valence shells. Adding an extra electron requires placing it in a higher energy orbital, necessitating energy input.

Among the noble gases, helium (He) has the smallest atomic size, leading to the highest positive electron gain enthalpy because the added electron would experience significant repulsion in the compact 1s orbital. Neon (Ne), with a larger atomic size than helium, has a slightly lower positive electron gain enthalpy. Krypton (Kr) and xenon (Xe), being even larger, have progressively lower positive electron gain enthalpies.

Therefore, the order from the least positive to the most positive electron gain enthalpy is:

He < Xe < Kr < Ne

This sequence indicates that helium has the least tendency to gain an electron, while neon has the highest.