Class 9 Science Chapter 3 Notes | Atoms and Molecules

परमाणु एवं अणु (Class 9 Science Notes)

Introduction | परिचय (Class 9)

हमारे आसपास की हर वस्तु छोटे-छोटे कणों से बनी होती है जिन्हें atoms और molecules कहा जाता है।
This chapter explains the basic structure of matter and chemical combinations……………… Class 9

इस chapter में हम atoms, molecules, laws of chemical combination, atomic mass और chemical formula के बारे में पढ़ेंगे।

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Laws of Chemical Combination ( Class 9 NCERT )

रासायनिक संयोजन के नियम

Chemical reactions always follow certain laws.

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1. Law of Conservation of Mass

द्रव्यमान संरक्षण का नियम Class 9

Statement:

Mass can neither be created nor destroyed in a chemical reaction.

Example:

If 5 g hydrogen reacts with 40 g oxygen, then total mass of water formed will be 45 g.

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2. Law of Constant Proportions (Class 9 NCERT )

स्थिर अनुपात का नियम

Statement:

A chemical compound always contains the same elements in fixed proportion by mass.

Example:

Water always contains hydrogen and oxygen in the ratio of 1:8 by mass.

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What is an Atom? | परमाणु क्या है?

An atom is the smallest particle of an element that can take part in a chemical reaction.

Important Points:

• Atoms are very small.
• They cannot be seen with naked eyes.
• Symbols are used to represent atoms.

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Symbols of Elements | तत्वों के प्रतीक Class 9 NCERT

Scientists use symbols for elements.

Element	Symbol
Hydrogen	H
Oxygen	O
Nitrogen	N
Carbon	C
Sodium	Na
Potassium	K
Iron	Fe

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Atomic Mass | परमाणु द्रव्यमान

The mass of an atom is called atomic mass.

Unit:

Atomic mass unit (u)

Example:

• Hydrogen = 1 u
• Carbon = 12 u
• Oxygen = 16 u

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What is a Molecule? | अणु क्या है?

A molecule is formed when two or more atoms combine chemically.

Examples:

• Oxygen molecule → O₂
• Water molecule → H₂O
• Carbon dioxide → CO₂

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Types of Molecules

1. Molecules of Elements

Made up of same type of atoms.

Examples:

• O₂
• H₂
• N₂

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2. Molecules of Compounds

Examples:

• H₂O
• CO₂
• NH₃

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Ions | आयन

Charged particles are called ions.

Types of Ions

Ion Type	Charge	Example
Cation	Positive	Na⁺
Anion	Negative	Cl⁻

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Writing Chemical Formulae ( Class 9 )

रासायनिक सूत्र लिखना

Chemical formula shows:

• Elements present
• Number of atoms

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Rules for Writing Formulae

1. Write positive ion first
2. Write negative ion second
3. Balance charges

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Examples of Chemical Formulae for Class 9

Compound	Formula
Water	H₂O
Carbon dioxide	CO₂
Sodium chloride	NaCl
Calcium chloride	CaCl₂
Magnesium oxide	MgO

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Molecular Mass | अणुभार

The sum of atomic masses of all atoms in a molecule is called molecular mass.

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Examples of Molecular Mass

Water (H₂O)

$2\times 1 + 16 = 18\,u$2×1+16=18u

Therefore, molecular mass of water = 18 u

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Carbon Dioxide (CO₂)

$12 + 2\times 16 = 44\,u$12+2×16=44u

Therefore, molecular mass of carbon dioxide = 44 u

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Mole Concept | मोल संकल्पना

A mole represents a fixed number of particles.

Avogadro Number

1 mole = $6.022 \times 10^{23}$6.022×1023 particles

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Formula Unit Mass

The sum of atomic masses of ions in an ionic compound is called formula unit mass.

Example:

NaCl = 23 + 35.5 = 58.5 u

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Important Definitions

Term	Definition
Atom	Smallest particle of an element
Molecule	Group of atoms chemically combined
Atomic Mass	Mass of an atom
Molecular Mass	Sum of atomic masses
Ion	Charged particle