Which of the given pairs would have a more negative electron-gain enthalpy: (1) O or F
(2) F or Cl?

Explanation

(1) O and F both belong the seonc period. As one moves from O to F, atomic size decreases and nuclear charge
increases, due to these factors, the incoming electron when enters the valence shell, the amount of energy liberated in case of F is more than that of O. again F. atom `(1s^(2)2s^(2)2p^(5))` accepts one electron to form `F^(-)` ion `(1s^(2)2s^(2)2p^(6))` which has a stable configuration similar to neon. however, O-atom when converts to `O^(-)` does not attain any stable confil-guration. thus energy released in much higher in going from F to `""^(-)F^(-)` than in going from O to `O^(-)`. So, the electron-gain enthalpy of F is much more negative than that of O.
(2) N/A.