Why F- has less energy than F?

jorgdv

It's a basic matter that atoms almost filling their upper band release energy when they become a negative ion, equivalent to say that they have a positive electron affinity. But I don't understand why the ionized state is less energetic than the originial one in this kind of atoms; having the same effective charge, one could think that since the Coulomb potential doesn't change, and there are more electrons, the energy should increase after the process. And I know that in that state more orbitals are filled with pairs of electrons and it's more "stable", but how does it influence and how could you get to that conclusion?

Thanks in advance.

jorgdv

Anyone? :(

DrDu

I fear I don't understand your question.

jorgdv

Why Fluor free energy when it captures an electron? since the potential is the same and the repulsion is greater, shouldn't you have to give energy to create F-?

DrDu

The extra electron gets attracted by the nucleus once it is within the mean distance of the other electrons. These become ineffective in screening the nuclear charge at these distances.

jorgdv

Do you mean that the new electron contributes to the screening, increasing the effective nuclear charge and decreasing the potential energy?

DrDu

No, I wanted to say that the other electrons won't completely screen the nuclear charge at small distances from the nucleus.

jorgdv

Alright, but that happens with or without the new electron, so it shouldn't affect the energy, right? Then why does the atom release energy?

DrDu

Because this unscreened nuclear charge attracts the electron.

jorgdv

Alright I think I got it, so due to the small distances of the other electrons to the nucleus, the effective charge would seem slightly positive to an electron far away because they don't screen it completely, right?