According to Dalton’s atomic theory, atoms of a given element are

Identical in mass and chemical properties

Different in mass and chemical properties

Identical in mass but different in chemical properties

Different in mass but identical in chemical properties

What are the symbols of elements made from according to Berzilius?

One or two letters of the name of the element

The atomic number of the element

What does a chemical formula of a compound show?

Its constituent elements and the number of atoms of each

According to Dalton’s atomic theory, atoms of different elements have

Identical masses and chemical properties

Different masses and chemical properties

Identical masses but different chemical properties

Different masses but identical chemical properties

In compounds formed with polyatomic ions, how is the number of ions indicated?

By enclosing the formula in brackets

NCERT Solutions Class 9 Science Chapter 3 – Atoms and Molecules

Name: Atoms and Molecules – Class 9 Science
Uploaded: 2026-04-15
Description: Matter is made of tiny indivisible particles called atoms, which combine to form molecules and ions. Elements have unique properties and symbols, while compounds have definite compositions. Atomic and molecular masses help in understanding chemical formulas and reactions.

Class 9 Science

Last Updated: 15 April 2026

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Chapter Introduction

Matter is made of tiny indivisible particles called atoms, which combine to form molecules and ions. Elements have unique properties and symbols, while compounds have definite compositions. Atomic and molecular masses help in understanding chemical formulas and reactions.

हिंदी में (Hindi)

पदार्थ सूक्ष्म अविभाज्य कणों, जिन्हें अणु कहा जाता है, से बना होता है, जो मिलकर अणु और आयन बनाते हैं। तत्त्वों के विशिष्ट गुण और प्रतीक होते हैं, जबकि यौगिकों की निश्चित संरचना होती है। आणविक और परमाणु द्रव्यमान रासायनिक सूत्र और प्रतिक्रियाओं को समझने में मदद करते हैं।

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Important Points

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Practice MCQ Questions (30 questions)

Q1. Who postulated that if we go on dividing matter, we shall get smaller and smaller particles?

Q2. Who suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further?

Q3. What is the name given to the smallest particles of matter by Maharishi Kanad?

Q4. What is the law of conservation of mass?

Q5. Who established the law of constant proportions?

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Tough Words (Glossary)

Divisibility

Meaning: Ability to be divided into smaller parts

अर्थ: विभाज्यता

"Ancient thinkers debated the divisibility of matter."

Philosopher

Meaning: A thinker who studies fundamental questions

अर्थ: दार्शनिक

"Greek philosophers proposed early atomic ideas."

Postulate

Meaning: To suggest or assume as a basis for reasoning

अर्थ: प्रतिपादन करना / मान लेना

"Kanad postulated the existence of Parmanu."

Doctrine

Meaning: A set of beliefs taught or held

अर्थ: सिद्धांत / मत

"Katyayama elaborated the doctrine of tiny particles."

Parmanu

Meaning: Smallest indivisible particle (ancient Indian term)

अर्थ: परमाणु (प्राचीन भारतीय संकल्पना)

"Kanad named the smallest particle Parmanu."

Atom

Meaning: Smallest unit of an element retaining its properties

अर्थ: परमाणु

"An atom is the building block of matter."

Indivisible

Meaning: Not able to be divided

अर्थ: अविभाज्य

"Democritus called atoms indivisible."

Validate

Meaning: To confirm with evidence

अर्थ: सत्यापित करना

"The ideas could not be validated until experiments improved."

Element

Meaning: Pure substance made of one kind of atom

अर्थ: तत्व

"Oxygen is an element."

Compound

Meaning: Substance formed by chemical combination of elements

अर्थ: यौगिक

"Water is a compound of hydrogen and oxygen."

Law of Conservation of Mass

Meaning: Mass is neither created nor destroyed in a chemical reaction

अर्थ: द्रव्यमान संरक्षण का नियम

"Burning follows the law of conservation of mass."

Law of Constant Proportions

Meaning: Elements in a compound are present in fixed mass ratios

अर्थ: नियत अनुपात का नियम

"Water always has hydrogen and oxygen in 1:8 by mass."

Definite

Meaning: Clearly fixed and not changing

अर्थ: निश्चित

"Compounds have definite composition."

Proportion

Meaning: A comparative relation in size or amount

अर्थ: अनुपात

"The proportion 14:3 is fixed in ammonia."

Atomic Theory

Meaning: Theory describing atoms as building blocks of matter

अर्थ: परमाणु सिद्धांत

"Dalton’s atomic theory explained the laws of combination."

Postulates

Meaning: Statements assumed to be true as a basis of theory

अर्थ: उपपत्तियाँ / प्रतिज्ञापन

"Learn the postulates of Dalton’s theory."

Weaver

Meaning: A person who weaves cloth

अर्थ: बुनकर

"Dalton was born in a weaver’s family."

Turning Point

Meaning: A decisive change in a situation

अर्थ: टर्निंग पॉइंट / मोड़

"The atomic theory was a turning point in chemistry."

Symbol (Chemical)

Meaning: One- or two-letter code for an element

अर्थ: रासायनिक चिह्न

"Fe is the symbol for iron."

IUPAC

Meaning: Body that standardises chemical names and symbols

अर्थ: आईयूपीएसी (रासायनिक नामकरण संस्था)

"IUPAC approves element names."

Uppercase

Meaning: Capital letter form

अर्थ: बड़े अक्षर

"The first letter of a symbol is uppercase."

Lowercase

Meaning: Small letter form

अर्थ: छोटे अक्षर

"The second letter, if present, is lowercase."

Atomic Mass

Meaning: Mass of an atom relative to carbon-12

अर्थ: परमाणु द्रव्यमान

"Chlorine has an atomic mass of about 35.5 u."

Atomic Mass Unit (u)

Meaning: 1/12 of the mass of a carbon-12 atom

अर्थ: परमाणु द्रव्यमान इकाई (u)

"Oxygen’s mass is expressed in atomic mass units."

Isotope

Meaning: Atoms of the same element with different masses

अर्थ: समस्थानिक

"Carbon-12 is the standard isotope for atomic masses."

Relative

Meaning: Considered in relation to something else

अर्थ: सापेक्ष

"Atomic masses are relative to carbon-12."

Aggregate

Meaning: To collect into a group or mass

अर्थ: एकत्र होना / समूह बनना

"Ions aggregate to form solids."

Molecule

Meaning: Two or more atoms chemically bonded

अर्थ: अणु

"An oxygen molecule contains two atoms."

Diatomic

Meaning: A molecule consisting of two atoms

अर्थ: द्विपरमाणुक

"O2 is a diatomic molecule."

Ozone

Meaning: A form of oxygen with three atoms (O3)

अर्थ: ओज़ोन

"Ozone protects us from UV radiation."

Atomicity

Meaning: Number of atoms in a molecule

अर्थ: परमाणुता

"The atomicity of ozone is three."

Ion

Meaning: A charged atom or group of atoms

अर्थ: आयन

"Salt contains positive and negative ions."

Cation

Meaning: A positively charged ion

अर्थ: धनायन

"Na+ is a common cation."

Anion

Meaning: A negatively charged ion

अर्थ: ऋणायन

"Cl− is a common anion."

Polyatomic Ion

Meaning: Charged group of several atoms

अर्थ: बहुपरमाणुक आयन

"Sulfate (SO4)2− is a polyatomic ion."

Valency

Meaning: Combining capacity of an element

अर्थ: संयोजकता

"Oxygen has a valency of two."

Chemical Formula

Meaning: Symbolic representation of a compound

अर्थ: रासायनिक सूत्र

"NaCl is the chemical formula of common salt."

Crossover Method

Meaning: Method of balancing valencies to write formulae

अर्थ: क्रॉसओवर विधि

"Use the crossover method to write MgCl2."

Binary Compound

Meaning: Compound of two different elements

अर्थ: द्विघटकी यौगिक

"HCl is a binary compound."

Formula Unit

Meaning: Simplest ratio of ions in an ionic compound

अर्थ: सूत्रक इकाई

"NaCl is a formula unit, not a molecule."

Molecular Mass

Meaning: Sum of atomic masses in a molecule

अर्थ: आणविक द्रव्यमान

"The molecular mass of CO2 is 44 u."

Formula Unit Mass

Meaning: Sum of atomic masses in an ionic formula unit

अर्थ: सूत्रक इकाई द्रव्यमान

"NaCl has a formula unit mass of 58.5 u."

Independently

Meaning: Without help; on its own

अर्थ: स्वतंत्र रूप से

"Most atoms do not exist independently."

Macroscopic

Meaning: Large enough to be seen with the naked eye

अर्थ: सामान्य आँखों से दिखने योग्य

"We study macroscopic properties of matter in labs."

Microscopic

Meaning: Too small to be seen with the naked eye

अर्थ: सूक्ष्म

"Atoms are microscopic entities."

Magnitude (Order of)

Meaning: Approximate size or scale of a quantity

अर्थ: आदेश / परिमाण का स्तर

"Millions of atoms fit across a paper’s thickness—an order-of-magnitude idea."

Latin Name

Meaning: Name derived from Latin language

अर्थ: लैटिन नाम

"Fe comes from the Latin name ‘ferrum’."

Allotrope

Meaning: Different structural forms of the same element

अर्थ: समरूपी रूप / अलॉट्रॉप

"Oxygen and ozone are allotropes."

Notation

Meaning: A system of symbols used to represent things

अर्थ: लेखन-पद्धति / संकेतन

"Chemical notation uses symbols like H, O, and Na."

Mass Ratio

Meaning: Proportion of masses of elements in a compound

अर्थ: द्रव्यमान अनुपात

"The mass ratio of H:O in water is 1:8."

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Ch 2: Is Matter Around Us Pure?

Ch 4: Structure of the Atom

NCERT Solutions Class 9 Science Chapter 3 – Atoms and Molecules

Chapter Introduction

Watch Video Lecture

Important Points

Practice MCQ Questions (30 questions)

Q1. Who postulated that if we go on dividing matter, we shall get smaller and smaller particles?

Q2. Who suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further?

Q3. What is the name given to the smallest particles of matter by Maharishi Kanad?

Q4. What is the law of conservation of mass?

Q5. Who established the law of constant proportions?

Q6. What is the ratio of the mass of hydrogen to the mass of oxygen in water?

Q7. According to Dalton’s atomic theory, atoms of a given element are

Q8. What is the building block of all matter?

Q9. What are the symbols of elements made from according to Berzilius?

Q10. What is the symbol for Sodium derived from its Latin name?

Q11. What is the standard reference for measuring atomic masses?

Q12. How do atoms of most elements exist?

Q13. What is the smallest particle of an element that can participate in a chemical reaction?

Q14. What do you call a group of atoms that are chemically bonded together?

Q15. What is a diatomic molecule of oxygen represented as?

Q16. What is the smallest particle of an element or a compound that shows all the properties of the substance?

Q17. What is the charge on an anion?

Q18. What is the charge on a cation?

Q19. What is the chemical formula of water?

Q20. What do you call a molecule of three oxygen atoms?

Q21. What is the valency of hydrogen?

Q22. What is the chemical formula for sodium chloride?

Q23. What does a chemical formula of a compound show?

Q24. What is the term for clusters of atoms that act as an ion?

Q25. What is the sum of the atomic masses of all atoms in a molecule called?

Q26. What is the sum of the atomic masses of all atoms in a formula unit of a compound called?

Q27. What is the term for a compound made up of two different elements?

Q28. According to Dalton’s atomic theory, atoms of different elements have

Q29. What is the combining capacity of an element known as?

Q30. In compounds formed with polyatomic ions, how is the number of ions indicated?