Which of the following pairs of elements would have a more negative electron gain enthalpy?
a. `O` or `F`, b.`F` or `Cl`

Explanation

a. Both `O`and `F` are in the `2nd` period. From `O` to `F`, the atomic size decreases and the effective nuclear charge increases. Both these factors tend to increase the attraction between the nucleus and the incoming electron and hence electron gain enthalpy becomes more negative.
Further, gain of one electron by `F` gives `F^(ө)` ion which has stable inert gas configuration while the gain of one electron by `O` gives `O^(ө)` ion which does not have stable inert gas configuration. Consequently, the energy released is much higher in going from `O rarr O^(ө)`. In other words, electron gain enthalpy of `F` is much more negative `(-328 kJ "mol"^(-1))` than that of oxygen `(-141 kJ "mol"^(-5))`.
b. In general, the electron gain enthalpy becomes less negative on moving down the group. But the electron gain enthalpy of chlorine `(-349 kJ mol^(-1))` is more negative than that of fluorine `(-328 kJ mol^(-1))`. This is due to the small size of `F` atom. Due to its small size, the repulsion between non-bonding electrons in the relatively compact `2p`-subshell is comparatively large and hence the incoming electron is not accepted with the same ease as is the case with larger `CI` atom. Consequently, electron gain enthalpy of `CI` is more negative than that of `F`.