1. O or F. Both O and F lie in 2e” period. As we move from O to F the atomic size decreases. Due to smaller size off nuclear charge increases. Further, gain of one electron by
F → F-
F- ion has inert gas configuration, While the gain of one electron by
O → O-
gives O ion which does not have stable inert gas configuration. Consequently, the energy released is much higher in going from
F → F-
than going from O → O- . In other words electron gain enthalpy off is much more negative than that of oxygen.-
2. The negative electron gain enthalpy of Cl (e.g. ∆H = – 349 mol-1) is more than that of F (e.g. ∆H = -328 U mol-1 ).
The reason for the deviation is due to the smaller size off. Due to its small size, the electron repulsion in the relatively compact 2p-subshell are comparatively large and hence the attraction for incoming electron is less as in the case of Cl.