Class 11 NEET Chemistry Classification Of Elements And Periodicity In Properties

B<C<N<O (increasing first ionisation enthalpy)
I<Br<Cl<F (increasing electron gain enthalpy)

The electron gain enthalpy of chlorine is more than fluorine because in it the added electron goes to the smaller energy level (n = 2) and hence suffers significant repulsion from the electrons already present in this shell.
The first ionisation enthalpy of Nitrogen is more than oxygen due to half-filled p orbital which makes the nitrogen more stable.