2.2.4: Shielding

• Electron shielding and penetration result in variations in effective nuclear charge (Z*) that depend on shell and subshell.
• Electrons within a multi-electron atom interact with the nucleus and with all other electrons and each electron experiences both attraction to the nucleus and repulsion from interactions with other electrons.
• The amount of positive nuclear charge experienced by any individual electron is the effective nuclear charge (Z*).
• Core electrons feel a Z* that is close to, but less than, Z.
• Valence electrons experience a Z* that is much less than Z.
• Shielding is the reduction of true nuclear charge (Z) to the effective nuclear charge (Z*) by other electrons in a multi-electron atom/ion.
• No single electron can completely shield a full unit of positive charge due to penetration.
• Core electrons shield valence electrons but valence electrons have little effect on the Z* of core electrons.
• Penetration describes the ability of an electron in a given subshell to penetrate into other shells and subshells to get close to the nucleus.
• An electron orbital's penetration affects its ability to shield other electrons and affects the extent to which it is shielded by other electrons.
• Slater's formula: Z*=Z-S
• Each other electron in the same group as the chosen electron contributes 0.35 to S.
• If the electron-of-interest is in a d or f subshell, every electron in groups to the left contributes 1.00 to S.
• If the electron-of-interest is in an s or p subshell, all electrons in the next lower shell (n-1) contribute 0.85 to S and all the electrons in even lower shells contribute 1.00 to S.
• S of a 1s electron is 0.3.
• When valence electrons experience less nuclear charge than core electrons, different electrons experience different magnitudes of attraction to the nucleus.
• The attraction of the nucleus to valence electrons determines the atomic or ionic size, ionization energy, electron affinity, and electronegativity.
• As we go down a column of the periodic table, the valence Z* increases.
• As we go across a period of the periodic table, Z* increases.
• As we go across the table in periods 1-3, the shell stays constant as Z increases and the subshell changes from s to p, resulting in a gradual increase in valence Z*.
• Going from an (n)s subshell to an (n-1)d subshell, there are relatively large increases in valence Z* and in going from an (n-1)d subshell to an (n)p subshell, there is a relatively large decrease in Z*.
• As we increase Z by one proton, going from one period to the next, there is a relatively large decrease in Z*.