The first ionisation enthalpy of nitrogen (Z=7) is greater than that of oxygen (Z =8) but the reverse is true for the second ionisation enthalpy. Explain why.

Explanation

Let us first consider the electric configuration of N (Z=7) and O(Z=8)
`N(Z=7) =1s^2, 2s^2, 2p^3`

Nitrogen has three electrons in p-suborbit (half Filled p-subshell).
These valence electrons are arranged as three singly occupied orbitals.
This is very stable arrangement thatit requires very high energy to remove an electron from its outer most shell.
`O(Z=8) =1s^2, 2s^2, 2p^4`

Oxygen has four electrons in p-subshell.
These valence electrons are arranged as one doubly occupied and two singly occupied orbitals.
Due to doubly occupied orbitals, it is easier to remove a n electron from its outermost shell.
Thus, the first ionisation enthalpy of nitrogen is greater than that of oxygen.
While this is reversed for the second ionisation enthalpy.
Nitrogen having only two occupied electrons easily loose an electron while oxygen having three electrons arranged as three singly occupied orbitals become more stable and require very high energy to remove an electron from its outermost shell.