Orbital energy shielding

Schreckenbach, G., 1999, The 57Fe NMR Shielding in Ferrocene Revisited. A Density-Functional Study of Orbital Energies, Shielding Mechanisms, and the Influence of the Exchange-Correlation Functional , J. Chem. Phys., 110, 11936. 

B—The d orbitals are shielded more efficiently than the s orbitals. Thus, the less shielded d orbitals do not fill as readily as s orbitals with similar energy. 

Shielding and penetration -> Qualitative understanding of orbital energies in atoms... 

The valence orbital (2s and 2p) energy levels of the fluorine atom are stabilized remarkably well by a large positive charge of the nucleus and the absence of shielding eifects by inner-shell electrons. The orbital energy level of the 2p lies at —18.6 eV. This is 5 eV lower than that of the proton s ls-orbital [6]. 

Electron-electron repulsions raise orbital energy and make electrons easier to remove. Repulsions have the effect of shielding electrons from the full nuclear charge, reducing it to an effective nuclear charge, Zgff. Inner electrons shield outer electrons most effectively. 

What is penetration How is it related to shielding Use the penetration effect to explain the difference in relative orbital energies of a 3p and a 3d electron in the same atom. 

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