Why is the electron affinity of fluorine anomalously low?

Question

Ellie Andrews · Accepted Answer

So we are looking at the enthalpy of the process:
#X(g) + e^(-) rarr X^(-)(g)# As you have noted, the electron affinity of #F#, is anomalously low; lower than that of the second row element. As physical scientists we should still quote the figures: #F, -328*kJ*mol^(-1)# #Cl,-348*kJ*mol^(-1)# #Br, -324*kJ*mol^(-1)# #I,-295*kJ*mol^(-1)# The electrons of atomic fluorine reside in smaller more compact orbitals. The resultant electron density is greater than its lower Group members. Given this fact, that the electron cloud of #Cl# is larger, and more diffuse than that of its first row analogue, an incoming electron experiences less electron-electron repulsion, even though the the effective nuclear charge is diminished with respect to the first row halogen. The same trend is observed for the first 2 members of the chalcogen series, #O#, and #S#. You should look at the figures.

Wyatt Adkins · Answer

undefined The anomalously low electron affinity of fluorine is primarily due to its small atomic size and high electron density, resulting in strong electron-electron repulsions in the incoming electron cloud, which reduces the energy gained from accepting an additional electron.