How does the Modern Periodic Law differ from Mendeleev’s Periodic Law?

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Modern Periodic Law vs Mendeleev’s Periodic Law

🔵 Step 1: Mendeleev’s Periodic Law

Mendeleev proposed his Periodic Law in 1869. He arranged elements in increasing order of their atomic masses.

"The physical and chemical properties of elements are periodic functions of their atomic masses."

🟢 Step 2: Modern Periodic Law

The Modern Periodic Law was proposed by Henry Moseley in 1913. It states that the properties of elements are periodic functions of their atomic numbers.

"The physical and chemical properties of elements are periodic functions of their atomic numbers."

🟠 Step 3: Main Differences

Basis	Mendeleev’s Law	Modern Periodic Law
Proposed by	Dmitri Mendeleev	Henry Moseley
Year	1869	1913
Basis	Atomic mass	Atomic number
Arrangement	Generally increasing atomic mass	Increasing atomic number
Isotopes	Could not be placed satisfactorily	All isotopes occupy the same position
Anomalous pairs	Some anomalous pairs existed	Anomalies are resolved by atomic number

🎬 3D-Style Comparison Animation

Mendeleev → Atomic Mass

Mass Mass Mass

---

Modern → Atomic Number

Z=1 Z=2 Z=3

🟣 Step 4: Why was the Modern Law needed?

Atomic number gives the number of protons in the nucleus and determines the electronic configuration of an element. Since chemical properties depend strongly on electronic configuration, atomic number provides a more fundamental basis for periodic classification.

Atomic Number → Electronic Configuration → Chemical Properties

✅ Final Answer

Mendeleev’s Periodic Law is based on atomic mass, whereas the Modern Periodic Law is based on atomic number.

Mendeleev: Properties ∝ Atomic Mass

Modern: Properties ∝ Atomic Number

🎯 Exam Point

Mendeleev → 1869 → Atomic Mass

Moseley → 1913 → Atomic Number