Why is the electron affinity of fluorine anomalously low?

1 Answer
anor277
Nov 8, 2015

So we are looking at the enthalpy of the process:
#X(g) + e^(-) rarr X^(-)(g)#

Explanation:

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.

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