• # question_answer 82)   Nitrogen has positive electron gain enthalpy whereas oxygen has negative. However, oxygen has lower ionisation enthalpy than nitrogen. Explain.

Nitrogen has extra stability due to half filled p-orbitals : $1{{s}^{2}},2{{s}^{2}}2p_{x}^{1}2p_{y}^{1}2p_{z}^{1}$ The stable atom resists the addition of extra electron, i.e., external energy is required. On account of this nitrogen has positive electron gain enthalpy. Oxygen has unsymmetrical configuration: $1{{s}^{2}},2{{s}^{2}}2p_{x}^{2}2p_{y}^{1}2p_{z}^{1}$ The extra electron can be added and the added electron comes under the influence of nuclear attraction. So, in this process, energy is released and thus, electron gain enthalpy of oxygen is negative. The ionisation enthalpy of nitrogen is higher on account of the stability of atom while in oxygen, the electron is lost easily as the atom tries to acquire stable configuration of $2{{p}^{3}}$ from $2{{p}^{4}}$ and hence, its ionisation enthalpy is lower than nitrogen.