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

Explanation

(i) O or F. Both O and Flie in 2nd period. As we move from O to F the atomic size decreases.
Due to smaller size of F nuclear charge increases.
Further, gain of one electron by
`FtoF^(-)` F-ion has inert gas configuration, While the gain of one electron by `OtoO^(-)`
gives `O^(-)` ion which does not have stable inert gas configuration. Consequently, the energy released is much higher in going from
`FtoF^(-)` than going from `OtoO^(-)`. In other words electron gain enthalpy of F is much more negative than that of oxygen.
(ii) The negative electron gain enthalpy of Cl(eg. `DeltaH= -349 kJ mol^(-1)`) is more than that of F(eg. `DeltaH= - 328 kJ mol^(-1)`).
The reason for the deviation is due to the smaller size of F. Due to its small size, the electron repulsions in the relatively compact 2p-subshell are comparatively large and hence the attraction for incoming electron is less as in the case of Cl.