Final answer:
Period 2 elements like nitrogen may exhibit endothermic electron affinity due to their unique electron configurations. Specifically, entering a half-filled orbital or adding an electron to a higher energy level requires more energy. In contrast, subsequent EAs can be exothermic when more stable electron configurations are formed.
Step-by-step explanation:
Elements like nitrogen (N) and other Period 2 elements sometimes have endothermic electron affinities (EA), which means they require energy to add an extra electron. This can be viewed as an exception to the general trend that EA is usually exothermic, particularly across a period as the effective nuclear charge increases. The explanation for this phenomenon lies in the specific electron configurations of these elements.
For example, Group 15 elements have a half-filled p-orbital and the incoming electron must pair up, which requires extra energy due to electron-electron repulsions. Therefore, the EA becomes endothermic. Similarly, Group 2 elements have a filled s-orbital, and an incoming electron must enter the higher energy p-orbital. For noble gases in Group 18, a completely filled valence shell means that any additional electron must be added to a new, higher energy level, which is unfavorable and thus endothermic.
Interestingly, subsequent EAs may be exothermic for some elements. The reason for this is that once an electron is added and the atom is negatively charged, adding another electron may be more favorable energetically, especially if it leads to a more stable electron configuration.