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.