Final answer:
Noble gases have positive electron gain enthalpy because their filled outer shells make them highly stable, and adding an electron requires energy, disrupting their stable octet configuration.
Step-by-step explanation:
The noble gases, such as helium, neon, argon, etc., display a positive electron gain enthalpy due to their filled outer subshell, which makes them highly stable. Adding an electron to these elements mean that it must enter a significantly higher energy level where it is loosely bound, resulting in an unfavorable process. Hence, it is difficult to add an electron to a noble gas without applying high pressure and temperature, and doing so disrupts their stable, full octet configuration. This contrasts sharply with other elements, particularly halogens like chlorine, which release energy when gaining an electron because it leads them closer to the stable electron configuration of the noble gases.
Furthermore, the electron affinity trends on the periodic table indicate that noble gases do not form stable anions. Their electron configurations resist adding or removing electrons as they are already in the most stable state. This is why noble gases have electron affinities greater than or equal to zero, depicting that energy must be supplied to attach an additional electron to them.