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

The basic difference in approach between Mendeleev’s Periodic Law and the Modern Periodic Law lies in the fundamental property used to classify the elements.

🔷 Mendeleev’s Periodic Law:

“The properties of elements are a periodic function of their atomic masses.”

• Mendeleev arranged elements in order of increasing atomic mass.

• He focused on grouping elements with similar chemical properties into the same columns (groups).

• In some cases, he even reversed the order of atomic masses to keep similar elements together.

• Example: Co (58.9) placed before Ni (58.7), based on properties.

🔷 Modern Periodic Law:

“The properties of elements are a periodic function of their atomic numbers.”

• Introduced after the discovery of atomic number by Moseley.

• Elements are arranged in order of increasing atomic number, which reflects the number of protons in the nucleus.

• This resolves anomalies in Mendeleev’s table and explains periodicity more accurately.

• The electronic configuration and repeating patterns are better understood through atomic number.

🔍 Key Differences:

✅ Conclusion:

Mendeleev’s approach was based on atomic mass, while the modern approach is based on atomic number, which is a more scientific and accurate method to explain the periodicity in properties of elements.