Question

Why is the electron affinity of fluorine anomalously low?

Answer 1

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.