The modern periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers. This replaced Mendeleev's law, which arranged elements by increasing atomic mass and led to anomalies such as the placement of argon before potassium. Moseley's work on X-ray spectra showed that atomic number, the number of protons, is the true fundamental property determining an element's position and properties, since it governs the electronic configuration. The option of atomic mass is the outdated Mendeleev criterion that produced inconsistencies. Atomic radius is a periodic property, not the basis of arrangement. The number of neutrons varies among isotopes of the same element and cannot define periodic position. Recognising atomic number as the organising principle is a core NCERT concept. Plausibility check: arranging by atomic number correctly places argon before potassium and resolves the mass-based anomalies, confirming atomic number as the proper basis.