Nitrogen has positive electron gain enthalpy whereas oxygen has negative. However, oxygen has lower ionisation enthalpy than nitrogen. Explain.

Answer

A

Explanation

Electronic configuration of `._(7)N=1s^(2), 2s^(2), 2p_(x)^(1), 2p_(y)^(1), 2p_(z)^(1)`. Nitrogen has stable configuration because p-orbital is half-filled. Therefore, addition of extra electron to any of the p-orbital requires energy.
Electronic configuration of `._(8)O=1s^(2), 2s^(2), 2p_(x)^(2), 2p_(y)^(1), 2p_(z)^(1)`. Oxygen has `2p^(4)` electrons, so process of adding an electron to the p-orbital is exothermic.
Oxygen has lower ionisation enthalpy than nitrogen because by removing one electron from 2p-orbital, oxygen acquires stable configuration, i.e., `2p^(3)`. On the other hand, in case of nitrogen it is not easy to remove one of the three 2p-electrons due to its stable configuration.