🧪 Periodic Classification of Elements – Complete Chapter Summary
Tags: Periodic Table, Elements, Chemistry Notes, Class 10 Science, Mendeleev, Modern Periodic Table
Category: Science / Chemistry Education
Reading Time: 6 minutes
🌟 Introduction
Have you ever wondered how scientists manage to study more than 118 elements found on Earth?
The answer lies in the Periodic Classification of Elements, one of the most brilliant ideas in chemistry.
It helps organize elements based on their atomic structure, properties, and relationships — making it easy to predict new elements and understand their behavior.
⚛️ Early Attempts to Classify Elements
1️⃣ Dobereiner’s Triads (1817)
- Proposed by Johann Dobereiner, a German scientist.
- Grouped elements with similar properties into sets of three called triads.
- The atomic mass of the middle element was approximately the average of the other two.
- Example:
Li (6.9), Na (23), K (39.1) → 23 ≈ (6.9 + 39.1) / 2
➡️ Limitation: Could not include all known elements.
2️⃣ Newlands’ Law of Octaves (1866)
- Proposed by John Newlands, an English scientist.
- Arranged elements in increasing order of atomic masses.
- Every eighth element showed properties similar to the first — like musical octaves (Do-Re-Mi-Fa-So-La-Ti).
- Example: Li and Na have similar properties.
➡️ Limitation: Worked only up to Calcium and failed for heavier elements.
🧠 Mendeleev’s Periodic Table (1869)
Russian chemist Dmitri Mendeleev arranged 63 known elements in order of increasing atomic mass.
He also left vacant spaces for undiscovered elements and predicted their properties accurately (like Scandium, Gallium, Germanium).
🧾 Mendeleev’s Periodic Law
“Properties of elements are a periodic function of their atomic masses.”
✅ Merits
- Helped discover new elements.
- Revised incorrect atomic masses.
- Included noble gases later without disturbing the table.
❌ Demerits
- Position of isotopes was unclear.
- Hydrogen’s place was confusing.
- Some elements were out of order when arranged strictly by mass.
🔬 Modern Periodic Table
In 1913, Henry Moseley discovered that the atomic number (Z) — not atomic mass — is the real identity of an element.
He proposed the Modern Periodic Law:
“Properties of elements are a periodic function of their atomic numbers.”
📊 Structure of the Modern Periodic Table
- 7 periods (horizontal rows)
- 18 groups (vertical columns)
- Divided into s-, p-, d- and f-blocks
- Metals are on the left, non-metals on the right, and metalloids lie along the zig-zag line.
⚙️ Periodic Trends in the Modern Table
🧩 1. Valency
- Depends on valence electrons (outermost shell electrons).
- Elements in the same group have the same valency.
- Example: Group 1 → valency 1, Group 17 → valency 1.
📏 2. Atomic Size
- Decreases from left → right in a period (increasing nuclear charge).
- Increases down a group (new shells added).
🔋 3. Metallic and Non-Metallic Character
- Metallic character → tendency to lose electrons.
- Non-metallic character → tendency to gain electrons.
- Metallic character increases down a group, decreases across a period.
💎 Fun Facts
- Hydrogen behaves like both an alkali metal and a halogen.
- Lanthanides and Actinides are placed separately at the bottom.
- Fluorine is the most reactive non-metal, while Francium is the most reactive metal.
🧠 Key Takeaways
✅ Mendeleev’s table was based on atomic mass, modern table on atomic number.
✅ Periodic table helps predict chemical behavior and valency.
✅ Properties of elements repeat periodically.
✅ Metals, nonmetals, and metalloids are arranged logically for easy study.
📚 Quick Revision Table
| Property | Across a Period | Down a Group |
|---|---|---|
| Atomic Size | Decreases | Increases |
| Metallic Character | Decreases | Increases |
| Non-Metallic Character | Increases | Decreases |
| Valency | Changes from 1 to 0 | Remains Same |