: In a period, on moving from left to right, the ionisation enthalpy increases which can be explained on the basis of increased nuclear charge and decrease in atomic radii. Both the factors increase the force of attraction of the electron towards the nucleus and consequently, more energy is needed to remove the valence electrons and thus the ionisation enthalpies increase. Thus, the order should be:

However, has stable electronic configuration, as a result of which, its ionisation energy is greater than that of oxygen and thus, the correct order is :