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

(i) O or F. Electron gain enthalpy of F is much more negative than that of O. This is because as we from O to F, the atomic size decreases and nuclear charge increases, thereby increase the attraction of the nucleus for the incoming electron and hence electron gain enthalpy becomes more negative. Moreover gain of one electron by F gives F^- ion which has stable inert gas configuration (1s²2s², p⁶).

(ii) F or Cl: In general, the electron gain enthalpy becomes less negative on moving down the group. But the electron gain enthalpy of chlorine is more negative than that of flourine. This is due to the small size of F atom. Due to its small size, the electron-electron repulsion in the relatively compact 2p-subshell are comparatively larger than those present in the bigger 3p-subshell of Cl. Therefore, chlorine has higher tendency to accept an addition electron than F.