Effects in Orbital Screening

The Is screening effect is caused by one of the electrons hiding or screening the full electrostatic view of the positive nucleus by the other Is electron. As the nuclear charge Z increases in heavier elements the 1 s orbitals are pulled in closer to the nucleus and the ability of one electron to screen out the other electron is greater in a smaller confined space of the Is orbital region. Thus we see the screening increases in the Clementi-Raimondi Is values as Z increases and the value decreases. 

Since there is a lot of interest in the C atom for organic molecules we should note that the best single value for the 2s orbital (1.6083) is higher than the value for the 2p orbitals (1.5679). This means that the energies of the 2s and 2p orbitals are not degenerate in the C atom (the 2p set is still degenerate) and in particular the lower exponent of the 2p orbitals means the 2p orbitals extend a little further out from the nucleus compared to the 2s orbital which is slightly more compact. 

Selected Clementi-Raimondi Best-Single Zeta (JQ Values for Atomic Orbitals 

The ( value for H is an optimized average for the H2 molecule and other hydrides. 

This lends added credibility to the concept that 2p pi-orbitals are held less tightly than sigma orbitals and that the pi-electrons can delocalize more easily. 

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