# Why is the electron affinity of fluorine anomalously low?

Nov 8, 2015

So we are looking at the enthalpy of the process:
$X \left(g\right) + {e}^{-} \rightarrow {X}^{-} \left(g\right)$

#### 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:

$F , - 328 \cdot k J \cdot m o {l}^{- 1}$
$C l , - 348 \cdot k J \cdot m o {l}^{- 1}$
$B r , - 324 \cdot k J \cdot m o {l}^{- 1}$
$I , - 295 \cdot k J \cdot m o {l}^{- 1}$

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 $C l$ 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.