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For example, take the 2s electron in lithium - is the reason that it's ionisation energy is lower than would be expected if "shielding" didn't occur at all because the 1s electrons shield the attractive coulombic force from the nucleus or because the 2s electron is repelled by the 1s electrons? Is it possibly a mixture of the two?

To me, shielding seems to be a model to help conceptualize what is actually happening; am I wrong about this? What is actually going on?

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The shielding model works as follows:

Shielding effect (source)

So it is a mixture of attraction and pushing from protons and non-valence electrons, in accord with the Coulomb's law. The prediction of the effective core charge is not an easy task - you will have to use quantum chemistry methods (e.g. Hartree-Fock) to predict those, like its done in this reference.

I think its best viewed as a pragmatic model to boil down the higher, not very graphic, quantum theories, to one number, the effective nuclear charge: $$ Z_\text{eff} = Z - S$$

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  • $\begingroup$ Why do electrons that are outside electron of interest have no effect? Surely they also repel the electron on interest TOWARDS the positively charged nucleus. $\endgroup$ – RobChem Jan 5 '15 at 14:41
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    $\begingroup$ Electrons in the same shell do have an effect on ionisation energy, but electrons in a higher shell are "spread out" and have only a small effect. You have to picture them as an electron cloud - which will push the electron of interest from all directions and therefor cancels itself for the most part. $\endgroup$ – John H. K. Jan 5 '15 at 15:03
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Its actually Coulombic repulsion between electrons and attraction between electron and protons.

It can be said shelding if you are considering the last electron. But if u consider any electron in the lower shells the electron in the upper shells will push the electron towards the nucleus and it won't be called shilding.

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