Final answer:
Noble gases such as helium, neon, and argon have positive electron gain enthalpy due to their filled outer subshells, which constitute a stable configuration that does not favor gaining additional electrons.
Step-by-step explanation:
Noble gases, which include elements like helium, neon, and argon, possess a unique quality in that their outer electron shell is fully occupied, creating a very stable electronic configuration. This complete valence shell corresponds to a full octet for most noble gases, which renders them largely unreactive. Thus, these gases exhibit positive electron gain enthalpy because they do not have a tendency to attract additional electrons; in fact, adding an electron would necessitate occupying a higher, less stable energy level, which is unfavorable. Since noble gases already possess a stable electron configuration, there is no inherent need to alter that configuration through chemical bonding, as seen in other elements striving to achieve the noble gas state.
When an electron is forced into a noble gas atom, it disrupts the stable energy state, resulting in positive electron gain enthalpy. The energy released when an electron is added to a neutral atom in the gaseous state is generally expected to be negative; however, for noble gases, this process requires an input of energy, making it endothermic and thus producing a positive value for electron gain enthalpy.