# Question #16078

Jan 28, 2015

$\text{Cl}$ has a higher electron affinity than $\text{F}$ because electronic repulsions are greater in the smaller $\text{F}$ atom.

#### Explanation:

Electron affinity is the enthalpy change when 1 mol of gaseous atoms each gain an electron to form 1 mol of gaseous ions.

It is the enthalpy change for:

${X}_{\left(g\right)} + {e}^{-} \rightarrow {X}_{\left(g\right)}^{-}$

For fluorine the electron affinity = $\text{-328 kJ·mol"^"-1}$

For chlorine the electron affinity= $\text{-349 kJ.mol"^"-1}$

So, we can see that the value for chlorine is larger and negative compared with fluorine.

Both $\text{Cl}$ and $\text{F}$ are quite happy to accept an electron to complete their octet and get to a lower energy level.

But $\text{F}$ is a smaller atom, so the electrons are crowded together more closely.

The electron repulsions are greater in the $\text{F}$ atom, so there is not as much energy available to be released when the electron is added.

Fluorine, since it's such a small atom, has a very high electron density, which means that the repulsion the incoming electron feels will diminish the attraction coming from the nucleus and thus reduce its electron affinity.