Abstract: Based on fundamental principles of quantum mechanics, this study systematically investigates the theoretical framework of the alternation rule of looseness and tightness and its distinctive significance in explaining periodic anomalies. The shielding constant is calculated using the modified Slater's rule, elucidating the underlying mechanism of the alternating “tight-loose-tight-loose” electron layer configuration. Through analysis of representative cases—including property variations among copper group elements and anomalous oxidation behaviors of p-block oxoacids—the explanatory power of this alternation rule is demonstrated. Furthermore, systematic comparative analysis of physicochemical parameters across main-group and transition elements validates the universal applicability of this rule.

Key words: Alternation rule of looseness and tightness, Effective nuclear charge, Shielding effect, Slater rule, Second periodicity