The ionisation enthalpy of elements varies across period and group. The ionisation enthalpy decreases down a group and increases as we move from left to right in a period. It is affected by the following parameters.

• Effective nuclear charge on the outermost electrons.

• Electron- electron repulsion force.

• Stability of the element due to half filled and fully filled orbitals.

But this trend is not true for every element for example beryllium and boron. Due to stable configuration of Be a very high first ionisation energy is required to remove an electron but in case of boron it has one electron in 2p¹ less energy is required.

Electronic configuration of Nitrogen is 1s² 2s²2p_x¹2p_y¹2p_z¹

Electronic configuration of Oxygen is 1s² 2s²2p_x²2p_y¹2p_z¹

As nitrogen has half filled stable p orbital therefore it has a positive electron gain enthalpy.

The ionisation enthalpy of oxygen is lower than that of Nitrogen as because when we remove one electron from oxygen then it easily donates it to attain half filled stability but in case of nitrogen it is difficult to remove one electron as it already has half filled stability and it would become unstable after that.