Slater's Rules:

1) Write the electron configuration for the atom using the following design;

(1s)(2s,2p)(3s,3p) (3d) (4s,4p) (4d) (4f) (5s,5p)

2) Any electrons to the right of the electron of interest contributes no shielding. (Approximately correct statement.)

3) All other electrons in the same group as the electron of interest shield to an extent of 0.35 nuclear charge units

4) If the electron of interest is an s or p electron: All electrons with one less value of the principal quantum number shield to an extent of 0.85 units of nuclear charge. All electrons with two less values of the principal quantum number shield to an extent of 1.00 units.

5) If the electron of interest is an d or f electron: All electrons to the left shield to an extent of 1.00 units of nuclear charge.

6) Sum the shielding amounts from steps 2 through 5 and subtract from the nuclear charge value to obtain the effective nuclear charge.

Examples:

Calculate Z^* for a valence electron in fluorine.

(1s²)(2s²,2p⁵)

Rule 2 does not apply; 0.35 · 6 + 0.85 · 2 = 3.8

Z^* = 9 – 3.8 = 5.2 for a valence electron.

Calculate Z^* for a 6s electron in Platinum.

(1s²)(2s²,2p⁶)(3s²,3p⁶) (3d¹⁰) (4s²,4p⁶) (4d¹⁰) (4f¹⁴) (5s²,5p⁶) (5d⁸) (6s²)

Rule 2 does not apply; 0.35 · 1 + 0.85 · 16 + 60 · 1.00 = 73.95

Z^* = 78 – 73.95 = 4.15 for a valence electron.

Shielding

The first ionization energy for hydrogen is 1310 kJ·mol^–1 while the first ionization energy for lithium is 520 kJ·mol^–1. The IE for lithium is lower for two reasons;

1) The average distance from the nucleus for a 2s electron is greater than a 1s electron;

2) The 2s¹ electron in lithium is repelled by the inner core electrons, so the valence electron is easily removed.

For reason #2 the inner core electrons shield the valence electron from the nucleus so the outer most electron only experiences an effective nuclear charge. In the case of the lithium the bulk of the 1s electron density lies between the nucleus and the 2s¹ electron. So the valence electron `sees' the sum of the charges or approximately +1. In reality the charge the valence electron experiences is greater than 1 because the radial distribution show their is some probabilty of finding the 2s electron close to the nucleus.

Reference: http://intro.chem.okstate.edu/WorkshopFolder/SlaterRule.html