Abstract
Slater's rules produce horizontal trends for effective nuclear charge Z *, which is the atomic number, Z, minus a screening constant, S. Using Slater's system, S is found by totaling the screening by all electrons except the one in question. The rules for calculating S have been modified from those of Slater to account for poorer shielding by d and f electrons. When this new value is divided by the atomic number, the result is defined as the "screening percentage", SP. When screening percentages are calculated across the periodic table, horizontal and vertical trends result. These trends provide an indicator of shielding and distance from the nucleus for valence electrons. The new rules for calculating S create vertical trends that mirror ionization energies (IE) and account for more subtle periodic anomalies such as the scandide and lanthanide contractions. An empirical system for relating IE with the screening percentage is included and the experimental IE data are compared to calculated values based on this new relationship. Finally, the horizontal and vertical trends in screening percentage are presented in a "shaded" periodic table suitable for freshman students. More advanced methods for depicting screening percentages are suggested for the advanced student.
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