This Chapter of Class 10 Science covers the topics like the Dobernier Triads, Newland’s Law of Octaves, Mendeleev Periodic Table, Modern Periodic Table, Trends in Modern Periodic Table & many more. important Concepts which will help you to prepare well for your Examinations.
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Main topics covered in CBSE Class 10 Science, Periodic Classification of Elements are:
- Dobernier Triads
- Newland’s Law of Octaves
- Mendeleev Periodic Table
- Drawbacks of Mendeleev Periodic table
- Modern Periodic Table
- Trends in Modern Periodic Table
Chapter- 5 (Periodic Classification of Elements)
Here’s a complete and easy-to-understand summary of Class 10 Science Chapter 5: Periodic Classification of Elements based on the CBSE NCERT curriculum:
Contents
- 1 🧪 Chapter 5 – Periodic Classification of Elements
- 2 📌 Summary Points:
🧪 Chapter 5 – Periodic Classification of Elements
🎯 Chapter Objectives:
- Understand why classification of elements is necessary.
- Learn about the historical attempts at classification.
- Understand Mendeleev’s Periodic Table.
- Study the Modern Periodic Table and trends across periods and groups.
🔹 1. Need for Classification
- Over 100 elements exist — classifying helps in:
- Studying their properties easily.
- Predicting properties of new elements.
- Understanding relationships among elements.
🔹 2. Early Attempts at Classification
a. Dobereiner’s Triads (1817)
- Grouped elements in sets of three with similar properties.
- Atomic mass of middle element ≈ average of first and third.
- Example: Li (7), Na (23), K (39)
✅ Limitation: Couldn’t apply to all elements.
b. Newlands’ Law of Octaves (1866)
- Every 8th element had similar properties (like musical octaves).
- Arranged in increasing atomic masses.
✅ Limitation: Valid only up to calcium. Beyond that, pattern failed.
🔹 3. Mendeleev’s Periodic Table (1869)
- Arranged elements by increasing atomic mass.
- Placed elements with similar properties in same groups.
🧠 Key Features:
- Groups (vertical): 8 groups (I to VIII)
- Periods (horizontal): 6 periods
- Left gaps for undiscovered elements (like Gallium, Germanium).
✅ Strengths:
- Helped predict new elements.
- Grouped elements with similar properties.
❌ Limitations:
- No fixed position for hydrogen.
- Anomalies in atomic masses (e.g., Co & Ni)
- Didn’t explain isotopes well.
🔹 4. Modern Periodic Table (by Henry Moseley, 1913)
- Elements arranged by increasing atomic number (Z).
- Solved issues in Mendeleev’s table.
🧩 Structure:
- 18 Groups (vertical columns)
- 7 Periods (horizontal rows)
🔁 Trends in the Periodic Table:
| Property | Across a Period | Down a Group |
|---|---|---|
| Atomic size | Decreases (more nuclear pull) | Increases (more shells) |
| Metallic character | Decreases (more nonmetallic) | Increases |
| Non-metallic character | Increases | Decreases |
| Valency | First increases, then decreases | Same within a group |
🔹 5. Position of Elements
- Metals: Left and center (Groups 1–12)
- Non-metals: Right side (Groups 14–18)
- Metalloids: Borderline elements (B, Si, As, etc.)
🧠 Important Terms
| Term | Definition |
|---|---|
| Valency | Number of electrons gained/lost/shared |
| Atomic size | Distance from nucleus to outermost shell |
| Ionization energy | Energy needed to remove an outer electron |
📘 NCERT-Intext & Exercise Questions
Make sure to practice:
- Trends in atomic size
- Periodic table structure
- Characteristics of periods/groups
- Comparing Mendeleev’s & Modern tables
📌 Summary Points:
- Modern periodic table is based on atomic number.
- Periods = 7, Groups = 18.
- Properties repeat periodically due to repeating electron configuration.
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