Clarification of forces involved in electron shielding

[BritKnight]

In my chemistry class we just started doing stuff with ionization energy, atomic radius, etc., and I've heard the phrase "electron shielding" tossed around a lot. When I tried to look it up online, most places use "shield" as the verb describing this process, which is not very helpful. The most specific explanation I have gotten so far is that shielding has to do with the electrons in inner shells repelling those in the outer shells, diminishing the attraction of the nucleus.

My question is: Why is it only the inner shells that "shield" the valence electrons? Wouldn't the electrons in the valence shell "shield" each other much more because they are much closer (although this doesn't seem to be the case looking at a graph of atomic radius wrt atomic number, which shows that the atomic radius increases much more when a new shell is added than when another valence electron in the same shell is added)?

Thanks a lot, BritKnight.

 

[mfb]

To get an effective shielding, the electrons should be "between" the nucleus and the orbital where you calculate the shielding. Inner electrons are better. Outer electrons have a part of their orbitals outside, where they do not contribute to the shielding.