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Tamil Nadu (TNBSE)Class 9 Science★ 4-5 marks in assessments

Periodic Classification of Elements

Samacheer Kalvi (TN State Board) Class 9 Science — Chemistry Chapter 12, Periodic Classification of Elements. The periodic table organizes all known chemical elements. Understanding groups, periods, and periodic trends (atomic size, electronegativity) helps predict the chemical behavior of elements and is crucial for advanced chemistry. Notes, worked examples, common mistakes, and quick revision.

⏱ 18 min readMedium difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1Summarise historical classifications (Dobereiner, Newlands, Mendeleev)
2State the Modern Periodic Law
3Describe the features of periods and groups in the Modern Periodic Table
4Identify periodic trends: atomic radius, valency, metallic character

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Why this chapter matters

The periodic table organizes all known chemical elements. Understanding groups, periods, and periodic trends (atomic size, electronegativity) helps predict the chemical behavior of elements and is crucial for advanced chemistry.

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Class 8 Science — Matter Around Us

Concept of elements and basic properties.

Periodic Classification of Elements — Class 9 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 9 Science, Chemistry — Chapter 12. The periodic table organizes all known chemical elements. Understanding groups, periods, and periodic trends (atomic size, electronegativity) helps predict the chemical behavior of elements and is crucial for advanced chemistry.

1. About this chapter

This chapter covers early periodic attempts, Mendeleev's periodic table, the Modern Periodic Table, and periodic properties.

2. Early Attempts at Classification

Dobereiner's Triads: Groups of 3 elements where the middle atomic mass is the average of the other two (e.g. Li, Na, K).
Newlands' Law of Octaves: Every eighth element has properties similar to the first (like musical notes).
Mendeleev's Periodic Table: Based on atomic mass. Left gaps for undiscovered elements (e.g. Eka-boron, Eka-aluminium).

3. The Modern Periodic Table

Modern Periodic Law: Properties of elements are periodic functions of their atomic numbers.
Structure: 7 horizontal rows (periods) and 18 vertical columns (groups).
Blocks: s-block (Groups 1-2), p-block (Groups 13-18), d-block (Groups 3-12, transition elements), f-block (lanthanides and actinides).

4. Periodic Trends

Atomic Radius: Decreases across a period, increases down a group.
Valency: Increases from 1 to 4 and then decreases to 0 across a period; remains same in a group.
Metallic Character: Decreases across a period, increases down a group.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Modern Periodic Law

Properties = f(Atomic Number)

Elements are arranged in increasing order of atomic numbers.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Confusing Mendeleev's periodic law with Modern periodic law.

✓ Mendeleev's law is based on atomic mass; the Modern periodic law is based on atomic number.

WATCH OUT

✗ Stating atomic size increases across a period.

✓ Atomic size decreases across a period (left to right) because the nuclear charge increases, pulling electrons closer.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Multiple Choice Questions

Questions on group properties, modern law, and periodic trends.

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Short Answer Questions

Newlands' octaves, Mendeleev's table anomalies, and modern periodic table features.

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Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· Concept

State the Modern Periodic Law.

▶ Show solution

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

Q2MEDIUM· Trends

Explain the periodic trend of atomic radius in periods and groups.

▶ Show solution

1. In periods (left to right), atomic radius decreases due to increase in nuclear charge pulling the valence shell closer. 2. In groups (top to bottom), atomic radius increases because new shells are added, increasing the distance from nucleus.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Mendeleev: Periodic function of atomic mass.
•Modern Periodic Table: Arranged by atomic number (Moseley's work).
•18 groups, 7 periods.
•Across a period: atomic radius decreases, metallic character decreases.
•Down a group: atomic radius increases, metallic character increases.

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 4-5 marks in assessments

Question type	Marks each	Typical count	What it tests
MCQ	1	1-2	Base concepts and definitions
Short Answer	2-3	1-2	Descriptive and application points

Prep strategy

Understand core definitions and solve standard textbook problems.
Review common mistakes to avoid losing easy marks.

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Materials Science

Periodic trends help scientists predict and synthesize new alloys and semiconductor materials with specific properties.

Chemical Industries

Helps select catalysts and reactants based on group characteristics and reactivity trends.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Write definitions precisely as defined in the textbook.
Draw neat, labeled diagrams for biology and physics chapters.

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Read advanced reference materials to explore concepts beyond the school syllabus.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

Class 9 Annual ExamsHigh

NTSE Stage 1Medium

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

There are 18 vertical columns called groups and 7 horizontal rows called periods.

Arranging three elements with similar properties in increasing atomic mass shows that the mass of the middle element is roughly the average of the other two.

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