Which of the following pairs of elements would have a more negative electron gain enthalpy? Explain(a) N or O  (b) F or Cl

Between N and O, O has a more negative electron gain enthalpy. In fact N has a positive ∆HEN which can be attributed to the 2p3 e– configuration of N(1s22s22p3) . The half filled 2p orbital provides very high stability to N and hence prevents anything that would disrupt it. Therefore,N requires energy input for accepting e– and disrupting that(∆HEN>0) . Meanwhile O has configuration 1s22s22p4 and accepts electron to release energy (∆HEN<0). Between F and Cl, Cl has a more negative∆HEN despite lower EN,which is attributed to the extremely small size of F(Z=9)which results in repulsion of e– approaching it.While Cl has a relatively larger size, which compensates for relatively lower EN than F. Final Answer: Hence, O and Cl are the correct alternatives for a and b respectively.