Question

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

Answer 1

For this we need to know that electron gain enthalpy is the amount of energy released from an isolated gaseous atom when an electron is added to its outermost shell and ionization enthalpy is the amount of energy required to remove an electron from its outermost shell in its gaseous state.

Complete answer:
The electron gain enthalpy decreases as we move down the group and whereas it increases on moving left to right across a period. Which also means that it will become less negative as we move down the group and more negative as we move across a period from left to right.
On the other hand, the ionization enthalpy increases when we move across a period from left to right and decreases as we move down the group in a period.
Now as we know that the atomic number of nitrogen is $7$ and the atomic number of oxygen is $8$ therefore, the outermost shell of nitrogen will contain $3$ electrons i.e. $2{p^3}$ and oxygen will have $4$ electrons in its outermost shell i.e. $2{p^4}$
An atom is highly stable if it has a fully filled or half-filled orbital which in this case is nitrogen with half-filled $\text{p - subshell}$, so it will not gain an electron easily because it is already very stable, gaining an electron will make nitrogen less stable. Also, as nitrogen is very stable it will require high energy to remove an electron from its outermost shell. Hence nitrogen will have a positive gain enthalpy and a high ionization energy.
On the other hand we see that as after removing an electron from the $\text{p - subshell}$ of oxygen it will become half-filled and hence more stable so the ionization enthalpy of oxygen is low which means on providing a small amount of energy oxygen will easily lose an electron and will gain stability as it will have half-filled orbital whereas it has a negative gain enthalpy because oxygen needs only two electrons to complete its octet therefore it will easily gain an electron.

Note:
The ionization enthalpy decreases as we move down the group because the number of shells keeps on increasing and because of that shielding effect also increases. On the other hand, electron gain enthalpy increases as we move across the group as the positive charge on the nucleus increases.