[Shadow]

Why does chlorine have the highest electron affinity? It is even bigger than that of fluorine. Why?

[Schrödinger]

All the more so because of the size. 10 people, small apartment? Or 10 people, large mansion?

[Shadow]

The size should cause the opposite effect. The bigger the size, the more electrons are in the shell, the bigger is the repulsion effect of electrons to the upcoming electron. Affinity should therefore decrease in groups. I need a deeper explanation.

[Stovn0611]

Most likely the increased number of protons in the nucleus of chlorine is enough to compensate for the extra repulsion from its greater number of electrons so it will pull harder on the electrons than fluorine

[Shadow]

Why is the affinity decreasing after chlorine towards astatium then?

[Schrödinger]

Because size is not the only thing that matters. When it comes to F and Cl, although F is more electronegative, the fact that F is very small means more interelectronic repulsion when you try to put another electron in, as compared to the more roomy Cl. But the same comparison based on just size, cannot be made for the other halogens as well. It is both electronegativity and size that go hand in hand. Cl is able to handle the excess electron because the repulsion is lesser as in the case of F. F is too small, that the energy due to repulsion is greater than for Cl. It's just an observed fact, and we try to make sense of it.

*Astatine, btw

[Shadow]

Thank you Schrodinger for the response.
I don't completely understand this. What effect causes the particle to be unstable when charge is spread over small distance? Better said, why is the interelectronic repulsion bigger, when there is a smaller number of electrons?

[Schrödinger]

Well it's just charge concentrated in a small area. That's why I gave you the example of crowding. It's not just the number of electrons. It's how many of them is packed in - to put it in layman terms - unit volume that matters.

[Shadow]

The charge of the electron spreads across the atom, if the atom is small, there will be more repulsion inside it between the electrons, if it is relatively big, the charge will be spread across bigger surface and the repulsion will be less. Did I understand this properly?

[Schrödinger]

Yes. But I'd like to reiterate that it's not just the size that matters, because as you said, we have other elements like Astatine that are much bigger yet their electron affinity is not as great as Cl. You can say Cl has the 'right' or optimum charge to size ratio or something on those lines.