Ionic radius question

[FeralisExtremum]

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So I'm going over Chad's lecture on periodic trends and I have a question. His explanation about atomic radii makes perfect sense to me: increases as you go down because you are adding shells and becoming larger. Decreases as you move left to right because Zeff (effective nuclear charge) increases - more nuclear charge but the same amount of shielding from the valence shell, thus greater attraction and a tighter, closer hold of the valence electrons.

When it comes to ionic radii, this may be more detail than I need to know, but it gets a bit confusing. Let's take the Cl atom and its ions, Cl+ and Cl- for example. Cl- is a larger radius than Cl, and Cl+ is a smaller radius. Why does removing or adding an electron have any effect on the radii? Isn't Zeff effectively exactly the same for the valence electrons, regardless of if we remove or add one? Why should it matter if we take one away or put one in if we are still in the same valence shell and experiencing the same Zeff?

The only explanation I can think of is that electrons add some "bulk" to the atomic radii so removing or adding them can alter it, but that bulk is negligible compared to the effect in radii from changes in Zeff. Is my understanding correct?

 

[shootingstar7]

I think the number of electrons does matter. If you have the same amount of positive charge to pull on less electrons, each of your electrons will feel a greater attraction to the nucleus.

 

[Troyvdg]

when you remove an electron, the number of protons is still the same, so there is more "pull" on the remaining electrons so to speak

 

[FeralisExtremum]

But isn't the pull experienced by each valence electron independent of the others? They are spaced away from each other and not shielding one another. Going by what Chad taught for atomic radii, the effective nuclear charge is independent for each valence electron, it's not as if that charge is divided among all the electrons.

EDIT: The answer appears to be the shielding effect (http://en.wikipedia.org/wiki/Shielding_effect). Apparently even within the same shell there is repulsion among the electrons that decreases the net nuclear force felt by an electron. So more electrons --> more shielding --> less effective nuclear force --> larger radii. Opposite applies for cations.