The modern periodic law states that the physical and chemical properties of elements are periodic functions of:

Modern (Moseley's) periodic law: properties of elements are periodic functions of their **atomic number** ($Z$). This replaced Mendeleev's original law which was based on atomic mass. Why atomic number wins: it explains chemistry better (electronic structure follows $Z$, not mass) and resolves Mendeleev's anomalies (the Ar/K and Te/I pairs whose mass-order disagrees with chemistry-order).