Question

What is the basic difference in approach between the Mendeleev's Periodic Law and the Modern Periodic Law?

Answer 1

Mendeleev’s Periodic Law

• Mendeleev arranged elements in order of their increasing atomic weights (atomic masses).
• His law stated: “The physical and chemical properties of the elements are periodic functions of their atomic weights.”
• So, the basic guiding parameter for classification was atomic weight.

Modern Periodic Law

• Modern classification is based on atomic number (number of protons), not atomic weight.
• Modern Periodic Law states: “The physical and chemical properties of the elements are periodic functions of their atomic numbers.”
• Atomic number is a more fundamental property and removes many anomalies present in Mendeleev’s table (e.g., position of Ar–K, Co–Ni, etc.).

Compact Statement

The basic difference in approach is the change in the basis of classification: Mendeleev → atomic weight; Modern → atomic number.