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

### Q.20:- Which of the following pairs of elements would have a more negative electron gain enthalpy? (i) O or F (ii) F or Cl

Answer:-
(i) O or F. Both O and F lie 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
F —> F
F~ ion has inert gas configuration, While the gain of one electron by
0->O
gives CT 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 of F is much more negative than that of oxygen.
(ii) The negative electron gain enthalpy of Cl (∆ eg H = – 349 kj mol-1) is more than that of F (∆ eg H = – 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.