Chapter 15 of 25

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Tamil Nadu (TNBSE)Class 9 Science★ 5-6 marks in assessments

Carbon and its Compounds

Samacheer Kalvi (TN State Board) Class 9 Science — Chemistry Chapter 15, Carbon and its Compounds. Carbon is the backbone of organic chemistry. Its unique properties (tetravalency, catenation) enable it to form millions of compounds, ranging from coal and fuels to plastics, medicines, and living tissues. Notes, worked examples, common mistakes, and quick revision.

⏱ 21 min readHard difficulty✓ Free · NCERT-aligned

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

1Explain catenation, tetravalency, and multiple bond formation in carbon
2Differentiate allotropes of carbon: diamond, graphite, fullerenes
3Classify hydrocarbons into alkanes, alkenes, and alkynes
4Identify functional groups and draw basic organic structures

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Why this chapter matters

Carbon is the backbone of organic chemistry. Its unique properties (tetravalency, catenation) enable it to form millions of compounds, ranging from coal and fuels to plastics, medicines, and living tissues.

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Class 9 Science — Atomic Structure

Valency of carbon and covalent bonding.

Carbon and its Compounds — Class 9 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 9 Science, Chemistry — Chapter 15. Carbon is the backbone of organic chemistry. Its unique properties (tetravalency, catenation) enable it to form millions of compounds, ranging from coal and fuels to plastics, medicines, and living tissues.

1. About this chapter

This chapter covers carbon bonding, catenation, allotropes, hydrocarbons (alkanes, alkenes, alkynes), and functional groups.

2. Bonding in Carbon and Catenation

Tetravalency: Carbon has 4 valence electrons and forms covalent bonds.
Catenation: Ability of carbon to link with other carbon atoms to form long chains or rings.
Allotropes: Different physical forms of carbon. Diamond (hardest, 3D), Graphite (layered, conducts electricity), Fullerene ( $C_{60}$ , hollow sphere).

3. Hydrocarbons

Compounds containing only carbon and hydrogen:

Saturated (Alkanes): Single bonds. General formula: $C_{n} H_{2 n + 2}$ (e.g. Methane, $C H_{4}$ ).
Unsaturated: Double or triple bonds.
- Alkenes: Double bonds. General formula: $C_{n} H_{2 n}$ (e.g. Ethene, $C_{2} H_{4}$ ).
- Alkynes: Triple bonds. General formula: $C_{n} H_{2 n - 2}$ (e.g. Ethyne, $C_{2} H_{2}$ ).

4. Homologous Series

A series of compounds having the same functional group where successive members differ by a $- C H_{2} -$ group and 14 amu mass.

Key formulas & results

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

General Formula of Alkanes

C_n H_(2n+2)

Saturated hydrocarbons (single bonds).

General Formula of Alkenes

C_n H_(2n)

Unsaturated hydrocarbons (double bond).

General Formula of Alkynes

C_n H_(2n-2)

Unsaturated hydrocarbons (triple bond).

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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

✗ Adding too many bonds to a carbon atom in structures.

✓ Carbon is tetravalent; it must have exactly four covalent bonds in any molecule. Check your structures by counting lines around each carbon.

WATCH OUT

✗ Calling graphite a poor electrical conductor.

✓ Unlike other non-metals, graphite has free delocalized electrons between its layered structure, making it a good conductor of electricity.

NCERT exercises (with solutions)

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

Multiple Choice Questions

Questions on allotropes, carbon bonding, and hydrocarbon formulas.

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Short Answer Questions

Graphite lubrication, tetravalency, isomerism, and saturated vs unsaturated tests.

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Practice problems

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

Q1EASY· Concept

Why does carbon form a large number of compounds?

▶ Show solution

1. Catenation: the ability to form strong covalent bonds with other carbon atoms to make long chains. 2. Tetravalency: four valence electrons allow bonding with four other atoms.

Q2MEDIUM· Structure

Write the names and chemical formulas of the first three members of the alkene homologous series.

▶ Show solution

Alkenes start with n=2 (double bond): 1. Ethene: C₂H₄. 2. Propene: C₃H₆. 3. Butene: C₄H₈.

5-minute revision

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

•Carbon has Z=6, valency 4 (tetravalency).
•Catenation allows carbon chains.
•Allotropes: Diamond, Graphite, Fullerene.
•Saturated: Alkanes ($C_nH_{2n+2}$).
•Unsaturated: Alkenes ($C_nH_{2n}$), Alkynes ($C_nH_{2n-2}$).

Tamil Nadu (TNBSE) marks blueprint

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

Typical chapter weightage: 5-6 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.

Fuels

Saturated hydrocarbons like methane (CNG), propane, and butane (LPG) are primary clean cooking and transport fuels.

Lubricants

Slippery graphite layers are used as dry lubricants in high-temperature machinery where oil would burn.

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

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1. Diamond: 3D tetrahedral structure, extremely hard, poor electrical conductor. 2. Graphite: 2D hexagonal layers, soft/slippery, good electrical conductor.

A family of organic compounds sharing the same functional group and chemical properties, where successive members differ by a -CH₂- unit.

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