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📚 NCERT & Assam Board | Class 9 Science
Chapter 3: Atoms and Molecules
Complete Study Guide
Notes · Questions & Answers · 50 MCQs · 15 Numericals · Memory Tricks · Mole Concept · Chemical Formulae — All in One Place!
50+
MCQs
15
Numericals
10+
Memory Tricks
15
FAQs
📋 Table of Contents
- Introduction & Learning Objectives
- Complete Chapter Notes — Laws, Dalton's Theory, Atoms, Molecules, Ions, Mole Concept
- Memory Tricks (10+)
- Important Formula Box
- Solved Numerical Problems (15)
- NCERT Textbook Q&A
- Assam Board Important Questions
- MCQ Practice Set (50 Questions)
- Previous Year Questions
- HOTS Questions
- Chapter Summary & Revision Sheet
- Common Mistakes
- FAQs (15)
🌟 Introduction to Atoms and Molecules
Why does this chapter matter? Let's find out!
Have you ever wondered why water always contains hydrogen and oxygen in the same ratio? Or how scientists know the exact weight of a single atom? 🤔 Welcome to one of the most fascinating chapters in Class 9 Science — Atoms and Molecules!
💡 Real-Life Connection
Everything around you — the air you breathe 🌬️, the water you drink 💧, the food you eat 🍎 — is made of tiny particles called atoms and molecules. Understanding these building blocks is the foundation of all Chemistry!
This chapter covers the laws of chemical combination, Dalton's atomic theory, atomic mass, molecular mass, and the powerful mole concept — a cornerstone of quantitative chemistry.
🏆 Exam Importance
This chapter carries high weightage in NCERT, SEBA (Assam Board), and competitive exams. Expect 5–8 marks in your school exams and 2–4 questions in NTSE / Olympiad.
✅ Learning Objectives
After studying this chapter, you will be able to:
- State and explain the Laws of Chemical Combination
- Describe Dalton's Atomic Theory and its postulates
- Define atom, write symbols of common elements
- Differentiate between molecules of elements and compounds
- Explain atomicity with examples
- Understand ions — cations and anions
- Write chemical formulae using valency
- Calculate atomic mass and molecular mass
- Apply the Mole Concept with Avogadro's number
- Solve numerical problems with confidence
📖 Complete Chapter Notes
⚗️ Topic 1: Laws of Chemical Combination
French chemist Antoine Lavoisier and Joseph Proust laid the foundation for modern chemistry through two fundamental laws.
📌 Law 1: Law of Conservation of Mass
📜 Definition
"Mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products."
— Antoine Lavoisier (1774)
Example: When hydrogen burns in oxygen to form water:
2H₂ + O₂ → 2H₂O
Mass of 2H₂ + O₂ = Mass of 2H₂O ✅
Experiment by Lavoisier: He burnt phosphorus in a sealed container. The total weight before and after burning was the same, proving mass is conserved.
🎯 Exam Point
"Mass is neither created nor destroyed" — learn this definition word-for-word. Often asked as 1-mark or fill-in-the-blank.
📌 Law 2: Law of Constant Proportions (Definite Proportions)
📜 Definition
"In a chemical compound, the elements are always present in definite proportions by mass, regardless of the source."
— Joseph Proust (1799)
Example: Water (H₂O) always contains Hydrogen and Oxygen in the ratio 1:8 by mass, whether it comes from a river, rain, or laboratory.
| Feature | Law of Conservation of Mass | Law of Constant Proportions |
|---|---|---|
| Scientist | Antoine Lavoisier | Joseph Proust |
| Year | 1774 | 1799 |
| About | Total mass in reaction | Ratio of elements in compound |
| Key Idea | Mass is conserved | Fixed composition of compounds |
| Example | 2H₂ + O₂ → 2H₂O (mass equal) | H₂O always has H:O = 1:8 |
🔬 Topic 2: Dalton's Atomic Theory
John Dalton (1766–1844) was an English chemist and physicist who proposed the first scientific theory about the atom in 1808.
👨🔬 Who was John Dalton?
John Dalton was a British chemist who proposed the atomic theory in 1808, providing a scientific explanation for the laws of chemical combination. He also suffered from color blindness, which is why it is sometimes called "Daltonism."
📋 Postulates of Dalton's Atomic Theory
- All matter is made of extremely tiny, indivisible particles called atoms.
- Atoms of the same element are identical in mass and properties.
- Atoms of different elements have different masses and properties.
- Atoms combine in the ratio of small whole numbers to form compounds.
- Atoms can neither be created nor destroyed in a chemical reaction.
- The relative number and kinds of atoms are constant in a compound.
⚠️ Limitations of Dalton's Atomic Theory
- Atoms are NOT indivisible — they have subatomic particles (proton, neutron, electron).
- Atoms of the same element can have different masses (isotopes).
- Does not explain allotropy (different forms of same element, e.g., diamond and graphite).
⚛️ Topic 3: Atoms
📜 Definition
Atom is the smallest particle of an element that retains the chemical identity of that element. Atoms may or may not exist independently.
Size of an atom: Atomic radius is measured in nanometres (nm). 1 nm = 10⁻⁹ m. Hydrogen, the smallest atom, has a radius of about 0.053 nm.
🔤 Symbols of Elements
Symbols are one or two letter representations of elements, usually derived from English or Latin names.
| Element | Symbol | Origin | Memory Trick |
|---|---|---|---|
| Hydrogen | H | English | H for Hydrogen — easy! |
| Oxygen | O | English | O like a balloon of gas 🎈 |
| Nitrogen | N | English | N = Nitrogen in air |
| Carbon | C | English | C for Coal (carbon) |
| Sodium | Na | Latin: Natrium | Na = NAtrium |
| Potassium | K | Latin: Kalium | K = KAlium |
| Iron | Fe | Latin: Ferrum | Fe = FErrum |
| Copper | Cu | Latin: Cuprum | Cu = CUprum |
| Silver | Ag | Latin: Argentum | Ag = AGentum |
| Gold | Au | Latin: Aurum | Au = AUrum |
| Lead | Pb | Latin: Plumbum | Pb = PlumBum |
| Mercury | Hg | Latin: Hydrargyrum | Hg = HydrarGyrum |
| Calcium | Ca | English | Ca = CAIcium |
| Chlorine | Cl | English | Cl = ChLorine |
| Aluminium | Al | English | Al = ALuminium |
| Sulphur | S | English | S = Sulphur |
| Phosphorus | P | English | P = Phosphorus |
| Magnesium | Mg | English | Mg = MaGnesium |
| Zinc | Zn | English/German | Zn = ZiNc |
🔗 Topic 4: Molecules
📜 Definition
A molecule is the smallest particle of a substance (element or compound) that can exist independently and retains all the physical and chemical properties of that substance.
Types of Molecules
| Type | Description | Examples |
|---|---|---|
| Molecules of Elements | Made of atoms of the same element | H₂, O₂, N₂, Cl₂, P₄, S₈ |
| Molecules of Compounds | Made of atoms of different elements | H₂O, CO₂, NH₃, HCl, NaCl |
Examples of Molecules of Compounds:
- H₂O (Water) — 2 hydrogen + 1 oxygen atom
- CO₂ (Carbon Dioxide) — 1 carbon + 2 oxygen atoms
- NH₃ (Ammonia) — 1 nitrogen + 3 hydrogen atoms
- HCl (Hydrochloric Acid) — 1 hydrogen + 1 chlorine atom
🔢 Topic 5: Atomicity
Atomicity is the number of atoms present in one molecule of an element.
| Atomicity | Name | Examples |
|---|---|---|
| 1 | Monoatomic | He (Helium), Ne (Neon), Ar (Argon), Kr (Krypton) — Noble gases |
| 2 | Diatomic | H₂, O₂, N₂, Cl₂, Br₂, I₂, F₂ |
| 3 | Triatomic | O₃ (Ozone), CO₂ (Carbon Dioxide) |
| 4 | Tetraatomic | P₄ (Phosphorus) |
| 8 | Polyatomic | S₈ (Sulphur) |
⚡ Topic 6: Ions
An ion is a charged particle formed when an atom gains or loses electrons.
| Type | Formation | Charge | Examples |
|---|---|---|---|
| Cation | Atom loses electrons | Positive (+) | Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺, H⁺, NH₄⁺ |
| Anion | Atom gains electrons | Negative (–) | Cl⁻, Br⁻, O²⁻, S²⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, OH⁻ |
💡 Ion Trick
Cat+ion = CAT has PAWS (positive) 🐱 | Ani-on = A for Anion = A for Attracted to positive = NEGATIVE ⚡
🧪 Common Ions and Their Valencies
| Ion | Symbol | Valency |
|---|---|---|
| Sodium | Na⁺ | 1 |
| Potassium | K⁺ | 1 |
| Calcium | Ca²⁺ | 2 |
| Magnesium | Mg²⁺ | 2 |
| Aluminium | Al³⁺ | 3 |
| Ammonium | NH₄⁺ | 1 |
| Chloride | Cl⁻ | 1 |
| Hydroxide | OH⁻ | 1 |
| Nitrate | NO₃⁻ | 1 |
| Sulphate | SO₄²⁻ | 2 |
| Carbonate | CO₃²⁻ | 2 |
| Phosphate | PO₄³⁻ | 3 |
| Oxide | O²⁻ | 2 |
✍️ Topic 7: Writing Chemical Formulae
Chemical formulae are written using the Criss-Cross Method (interchanging valencies).
📝 Steps for Writing Chemical Formula
1. Write the symbols of the elements/ions side by side.2. Write the valency of each ion below its symbol.
3. Criss-cross the valencies (exchange them).
4. Simplify if possible (use lowest ratio).
5. Write the formula with subscripts.
| Compound | Ions | Valencies | Formula |
|---|---|---|---|
| Sodium Chloride | Na⁺, Cl⁻ | 1, 1 | NaCl |
| Magnesium Chloride | Mg²⁺, Cl⁻ | 2, 1 | MgCl₂ |
| Calcium Oxide | Ca²⁺, O²⁻ | 2, 2 → 1:1 | CaO |
| Aluminium Oxide | Al³⁺, O²⁻ | 3, 2 | Al₂O₃ |
| Sodium Sulphate | Na⁺, SO₄²⁻ | 1, 2 | Na₂SO₄ |
| Calcium Carbonate | Ca²⁺, CO₃²⁻ | 2, 2 → 1:1 | CaCO₃ |
| Ammonium Chloride | NH₄⁺, Cl⁻ | 1, 1 | NH₄Cl |
| Aluminium Sulphate | Al³⁺, SO₄²⁻ | 3, 2 | Al₂(SO₄)₃ |
⚖️ Topic 8: Atomic Mass
📜 Definition
Atomic Mass is the relative mass of an atom compared to 1/12 of the mass of a Carbon-12 atom.
Unit: u (unified atomic mass unit) or amu. 1 u = 1.66 × 10⁻²⁷ kg
| Element | Symbol | Atomic Mass (u) |
|---|---|---|
| Hydrogen | H | 1 |
| Helium | He | 4 |
| Carbon | C | 12 |
| Nitrogen | N | 14 |
| Oxygen | O | 16 |
| Sodium | Na | 23 |
| Magnesium | Mg | 24 |
| Aluminium | Al | 27 |
| Phosphorus | P | 31 |
| Sulphur | S | 32 |
| Chlorine | Cl | 35.5 |
| Calcium | Ca | 40 |
| Iron | Fe | 56 |
| Copper | Cu | 63.5 |
| Zinc | Zn | 65 |
🧮 Topic 9: Molecular Mass
📜 Definition
Molecular Mass = Sum of atomic masses of all atoms in one molecule of the substance.
Examples:
- Water (H₂O) = 2×1 + 1×16 = 2 + 16 = 18 u
- CO₂ = 12 + 2×16 = 12 + 32 = 44 u
- NH₃ = 14 + 3×1 = 17 u
- NaCl = 23 + 35.5 = 58.5 u
- H₂SO₄ = 2×1 + 32 + 4×16 = 2 + 32 + 64 = 98 u
ℹ️ Formula Mass
For ionic compounds like NaCl, we use Formula Mass (not molecular mass) since they don't exist as discrete molecules.
🔢 Topic 10: Mole Concept
📜 Definition of Mole
One mole is the amount of substance that contains as many elementary particles (atoms, molecules, ions) as there are atoms in exactly 12 grams of Carbon-12.
This number is Avogadro's Number (Nₐ) = 6.022 × 10²³
- 1 mole of atoms = 6.022 × 10²³ atoms
- 1 mole of molecules = 6.022 × 10²³ molecules
- Molar Mass = Atomic/Molecular mass expressed in grams
- 1 mole of water = 18 g of water = 6.022 × 10²³ molecules of water
🎯 Remember This Triangle
Mass ↔ Mole ↔ Number of ParticlesMoles = Mass ÷ Molar Mass
Number of Particles = Moles × 6.022 × 10²³
🧠 Memory Tricks (10+)
Remember everything easily — these tricks actually work!
🎵 Trick 1: Diatomic Molecules — "HONClBrIF"
Have Our Neat Class, Brown Is Fine!
Hydrogen · Oxygen · Nitrogen · Clorine · Bromine · Iodine · Fluorine
🌍 Trick 2: Latin Symbol Metals — "Na K Fe Cu Ag Au Pb Hg"
Never Keep Fancy Cars. Always Ask People Home
Natrium · Kalium · Ferrum · Cuprum · Agentum · Aurum · Pbum · Hg
🔢 Trick 3: Atomicity Sequence
Mono-Di-Tri-Tetra-Poly = 1-2-3-4-Many
Noble gases = Mono | O₂,H₂,N₂ = Di | O₃ = Tri | P₄ = Tetra | S₈ = Poly
⚡ Trick 4: Cation vs Anion
CATion = CATch electrons (loses them → positive) | ANion = ANgry with protons (gains electrons → negative)
📐 Trick 5: Valency of Common Ions
"NaK has 1, MgCa has 2, Al has 3" — Group = Valency
Group 1 → Valency 1 | Group 2 → Valency 2 | Group 13/3 → Valency 3
🔬 Trick 6: Avogadro's Number
6.022 × 10²³ = "Six Oh Two Two, Times Ten to Twenty-Three"
Pronounced: "six point oh-two-two into ten to the power twenty-three" — Amedeo Avogadro
🧪 Trick 7: Mole Formula Triangle
MOLES = MASS ÷ MOLAR MASS → "MMM" (Mass/Molar Mass = Moles)
🗓️ Trick 8: Key Atomic Masses to Memorize
H=1, C=12, N=14, O=16, Na=23, Mg=24, Al=27, S=32, Cl=35.5, Ca=40, Fe=56, Cu=63.5
Song: "1, 12, 14, 16, 23, 24, 27, 32..." — chant like multiplication tables!
📜 Trick 9: Laws of Chemical Combination — "LP" (Lavoisier-Proust)
Lavoisier = Loses nothing (Conservation of Mass) | Proust = Proportions stay (Constant Proportions)
✍️ Trick 10: Criss-Cross Formula Writing
Write → Valency below → X across → Simplify → Done!
Example: Al(3) O(2) → Al₂O₃. Always simplify: Ca(2) O(2) → CaO (not Ca₂O₂)
🌟 Trick 11: Dalton's Postulates — "MICNA"
Matter Is Composed of tiny, Non-divisible Atoms
M=Matter made of atoms | I=Identical atoms same element | C=Combine in whole numbers | N=Neither created/destroyed | A=Atoms vary for elements
📐 Important Formula Box
Highlighted formulas for quick revision
Molecular Mass
Molecular Mass = Sum of atomic masses of all atoms in the molecule
Mole Formula
Number of Moles (n) = Given Mass (m) ÷ Molar Mass (M)
Number of Particles
Number of Particles = n × Nₐ = (m/M) × 6.022 × 10²³
Mass from Moles
Mass (g) = Number of Moles × Molar Mass
Moles from Particles
Moles = Number of Particles ÷ 6.022 × 10²³
Avogadro's Number
Nₐ = 6.022 × 10²³ mol⁻¹
Relative Atomic Mass
Relative Atomic Mass = Mass of atom ÷ (1/12 × Mass of C-12 atom)
🧮 Solved Numerical Problems (15)
Step-by-step solutions for exam practice
Q1. Calculate the molecular mass of water (H₂O).
Given: H₂O. Atomic masses: H = 1 u, O = 16 u
Step 1: Count atoms: 2 H atoms, 1 O atom
Step 2: Mass = 2(1) + 1(16) = 2 + 16
✅ Molecular Mass of H₂O = 18 u
Q2. Calculate the molecular mass of CO₂.
Given: CO₂. Atomic masses: C = 12 u, O = 16 u
Step: 1(12) + 2(16) = 12 + 32
✅ Molecular Mass of CO₂ = 44 u
Q3. Calculate the molecular mass of H₂SO₄ (Sulphuric Acid).
Atoms: H=2, S=1, O=4
Step: 2(1) + 1(32) + 4(16) = 2 + 32 + 64
✅ Molecular Mass of H₂SO₄ = 98 u
Q4. Calculate the formula mass of NaCl (table salt).
Atoms: Na=1, Cl=1. Atomic masses: Na=23, Cl=35.5
Step: 23 + 35.5
✅ Formula Mass of NaCl = 58.5 u
Q5. Calculate the molar mass of NH₃ (Ammonia).
Atoms: N=1, H=3. N=14, H=1
Step: 14 + 3(1) = 14 + 3
✅ Molar Mass of NH₃ = 17 g/mol
Q6. How many moles are in 36 g of water?
Given: Mass = 36 g, Molar mass of H₂O = 18 g/mol
Formula: n = m / M = 36 / 18
✅ Number of moles = 2 mol
Q7. Calculate the number of molecules in 44 g of CO₂.
Step 1: Molar mass of CO₂ = 44 g/mol
Step 2: Moles = 44/44 = 1 mol
Step 3: Number of molecules = 1 × 6.022 × 10²³
✅ Number of molecules = 6.022 × 10²³
Q8. What is the mass of 0.5 mole of oxygen gas (O₂)?
Step 1: Molar mass of O₂ = 2 × 16 = 32 g/mol
Step 2: Mass = moles × molar mass = 0.5 × 32
✅ Mass = 16 g
Q9. How many atoms are present in 2 moles of hydrogen?
Step 1: 1 mole = 6.022 × 10²³ atoms
Step 2: 2 moles = 2 × 6.022 × 10²³
✅ Number of atoms = 12.044 × 10²³ = 1.2044 × 10²⁴ atoms
Q10. Calculate the number of moles in 6.022 × 10²² atoms of sulphur.
Formula: n = Number of atoms / Nₐ
Step: n = 6.022 × 10²² / 6.022 × 10²³ = 0.1
✅ Number of moles = 0.1 mol
Q11. What is the molecular mass of Ca(OH)₂ (Calcium Hydroxide)?
Atoms: Ca=1, O=2, H=2. Ca=40, O=16, H=1
Step: 40 + 2(16) + 2(1) = 40 + 32 + 2
✅ Molecular Mass = 74 u
Q12. Find the mass of 3 moles of aluminium atoms. (Al = 27)
Mass = moles × molar mass = 3 × 27
✅ Mass = 81 g
Q13. Calculate the molecular mass of HNO₃ (Nitric Acid).
Atoms: H=1, N=1, O=3. H=1, N=14, O=16
Step: 1 + 14 + 3(16) = 1 + 14 + 48
✅ Molecular Mass of HNO₃ = 63 u
Q14. How many moles of NaOH are present in 40 g? (Na=23, O=16, H=1)
Molar mass of NaOH: 23 + 16 + 1 = 40 g/mol
n = 40 / 40 = 1
✅ 1 mole of NaOH
Q15. Calculate the number of atoms in 0.1 mole of carbon.
Atoms = moles × Nₐ = 0.1 × 6.022 × 10²³
✅ Number of atoms = 6.022 × 10²² atoms
📚 NCERT Textbook Questions & Answers
All important NCERT exercise questions with detailed answers
📌 In-Text Questions
Q1. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g of water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.
Solution: Mass of reactants = 5.3 + 6 = 11.3 g. Mass of products = 2.2 + 0.9 + 8.2 = 11.3 g. Since mass of reactants = mass of products, the law of conservation of mass is verified. ✅
Q2. Hydrogen and oxygen combine in the ratio 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Solution: Ratio of H:O = 1:8. For 3 g of H, mass of O = 3 × 8 = 24 g.
Q3. Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?
The postulate: "Atoms can neither be created nor destroyed in a chemical reaction" is the result of the law of conservation of mass.
Q4. Which postulate of Dalton's atomic theory can explain the law of definite proportions?
The postulate: "Atoms combine in the ratio of small whole numbers to form compounds" and "the relative number and kinds of atoms are constant in a compound" explain the law of constant proportions.
Q5. Define atomic mass unit.
One atomic mass unit (u) is defined as exactly 1/12 of the mass of one atom of Carbon-12 isotope. 1 u = 1.66 × 10⁻²⁷ kg.
Q6. Why is it not possible to see an atom with naked eyes?
Atoms are extremely small, with radii in the order of nanometres (10⁻⁹ m). The smallest atom (hydrogen) has a radius of about 0.053 nm, far smaller than the resolution of naked eyes or even ordinary microscopes.
📌 NCERT Exercise Questions
Q1. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound.
% Boron = (0.096 / 0.24) × 100 = 40%. % Oxygen = (0.144 / 0.24) × 100 = 60%.
Q2. Calculate the molecular mass of: (a) H₂O (b) CO₂ (c) CH₄
(a) H₂O = 2(1) + 16 = 18 u
(b) CO₂ = 12 + 2(16) = 44 u
(c) CH₄ = 12 + 4(1) = 16 u
(b) CO₂ = 12 + 2(16) = 44 u
(c) CH₄ = 12 + 4(1) = 16 u
Q3. Calculate the formula unit masses of: ZnO, Na₂O, K₂CO₃ (given atomic masses: Zn=65, Na=23, K=39, C=12, O=16)
ZnO = 65 + 16 = 81 u
Na₂O = 2(23) + 16 = 46 + 16 = 62 u
K₂CO₃ = 2(39) + 12 + 3(16) = 78 + 12 + 48 = 138 u
Na₂O = 2(23) + 16 = 46 + 16 = 62 u
K₂CO₃ = 2(39) + 12 + 3(16) = 78 + 12 + 48 = 138 u
Q4. If one mole of carbon atoms weighs 12 g, what is the mass in grams of 1 atom of carbon?
Mass of 1 atom = 12 g ÷ 6.022 × 10²³ = 1.99 × 10⁻²³ g
Q5. Which has more number of atoms — 100 g of sodium or 100 g of iron?
Moles of Na = 100/23 = 4.35 mol
Moles of Fe = 100/56 = 1.79 mol
Since Na has more moles, 100 g of Sodium has more atoms.
Moles of Fe = 100/56 = 1.79 mol
Since Na has more moles, 100 g of Sodium has more atoms.
🎓 Assam Board Important Questions
SEBA — Questions & Answers by mark weight
📝 1-Mark Questions (Very Short Answer)
- What is the symbol of sodium? — Na
- Who proposed the atomic theory? — John Dalton (1808)
- What is the atomicity of ozone (O₃)? — 3 (Triatomic)
- Name the unit of atomic mass. — u (unified atomic mass unit)
- What is Avogadro's number? — 6.022 × 10²³
- What is the molecular mass of O₂? — 32 u
- Which element has the symbol Fe? — Iron
- What is the formula of water? — H₂O
- State the law of conservation of mass. — Mass is neither created nor destroyed in a chemical reaction.
- What is the atomicity of phosphorus? — 4 (P₄)
- Define one mole. — Amount of substance containing 6.022 × 10²³ particles.
- What is the valency of aluminium? — 3
- Give one example of a monoatomic element. — Helium (He)
- What is the symbol of potassium? — K
- What does Na stand for? (Latin name) — Natrium
📝 2-Marks Questions
- Distinguish between atoms and molecules. — Atoms are the smallest units of an element; molecules are the smallest units of a substance that can exist independently.
- State the law of constant proportions with an example. — Elements in a compound are always present in fixed ratio by mass. E.g., H₂O always has H:O = 1:8.
- What are cations and anions? Give one example each. — Cations are positively charged ions (e.g., Na⁺). Anions are negatively charged ions (e.g., Cl⁻).
- Write the chemical formula of Magnesium Chloride. Show working. — Mg²⁺ and Cl⁻ → MgCl₂ (by criss-cross method)
- What is atomicity? Classify O₂, P₄, He, and O₃ based on atomicity. — He=Monoatomic, O₂=Diatomic, O₃=Triatomic, P₄=Tetraatomic
- Calculate the molecular mass of NH₃. — N(14) + 3×H(1) = 17 u
- What is a mole? How many particles are in one mole? — 1 mole = 6.022 × 10²³ particles (Avogadro's number)
- What were the postulates of Dalton that support the laws of chemical combination? — "Atoms are indivisible" supports conservation of mass; "fixed ratio combination" supports constant proportions.
- Write the name and formula of 3 compounds of aluminium. — Al₂O₃ (Aluminium oxide), AlCl₃ (Aluminium chloride), Al₂(SO₄)₃ (Aluminium sulphate)
- State two limitations of Dalton's atomic theory. — (1) Atoms are divisible (subatomic particles exist). (2) Same element can have atoms of different masses (isotopes).
- What is relative atomic mass? Give example. — Mass of atom relative to 1/12 of C-12 atom. E.g., Oxygen's relative atomic mass = 16.
- How many moles are present in 46 g of sodium? — n = 46/23 = 2 moles
- Write the symbols of: Gold, Silver, Iron, Lead, Mercury. — Au, Ag, Fe, Pb, Hg
- What is the formula mass of CaCO₃? — Ca(40) + C(12) + 3×O(16) = 40+12+48 = 100 u
- What do you mean by formula unit? — For ionic compounds, the formula unit represents the simplest ratio of ions (e.g., NaCl has 1 Na⁺ and 1 Cl⁻).
📝 3-Marks Questions
- State and explain the law of conservation of mass with an example. Mass of reactants = mass of products. Example: 2H₂ + O₂ → 2H₂O; masses balance.
- Write all postulates of Dalton's atomic theory. Six postulates about atoms, identity, combining, creation/destruction.
- Explain how to write the chemical formula of Al₂O₃ using the criss-cross method. Al(3) O(2) → criss-cross → Al₂O₃.
- Define mole. Calculate the number of atoms in 12 g of carbon. 12 g C = 1 mole = 6.022 × 10²³ atoms.
- What are ions? Explain cations and anions with examples and formation. Ions form when atoms gain/lose electrons. Cation (+): Na loses 1e⁻ → Na⁺. Anion (–): Cl gains 1e⁻ → Cl⁻.
- Calculate the molecular mass of H₂SO₄ and HCl. H₂SO₄ = 98 u; HCl = 1+35.5 = 36.5 u.
- What is Avogadro's number? A sample contains 3.011 × 10²³ molecules of O₂. Find its mass. Moles = 3.011×10²³/6.022×10²³ = 0.5 mol. Mass = 0.5×32 = 16 g.
- Differentiate between molecules of elements and molecules of compounds with examples. Element molecules: H₂, O₂, N₂ (same atoms). Compound molecules: H₂O, CO₂ (different atoms).
- Write the formulae and names of compounds formed from: Na⁺ and SO₄²⁻, Ca²⁺ and NO₃⁻, NH₄⁺ and Cl⁻. Na₂SO₄ (Sodium sulphate), Ca(NO₃)₂ (Calcium nitrate), NH₄Cl (Ammonium chloride).
- State the law of constant proportions and verify it for water from any two different sources. H:O ratio is always 1:8 in water from any source, verifying Proust's law.
- Explain atomicity with examples of monoatomic, diatomic, triatomic, and polyatomic molecules. See Topic 5 table above.
- How many moles of water are formed when 4 moles of H₂ react with 2 moles of O₂? 2H₂ + O₂ → 2H₂O. 4 moles H₂ → 4 moles H₂O. Answer: 4 moles H₂O.
- Calculate the % composition of elements in H₂O. % H = 2/18 × 100 = 11.1%; % O = 16/18 × 100 = 88.9%.
- Describe how Lavoisier's experiment proved the law of conservation of mass. Burnt tin in sealed container — total mass unchanged before and after reaction.
- What are polyatomic ions? Give three examples with charge. Ions made of multiple atoms: SO₄²⁻, NO₃⁻, CO₃²⁻, NH₄⁺, PO₄³⁻.
📝 5-Marks Questions (Long Answer)
- State and explain both laws of chemical combination. Give one example and experiment for each. Write a comparison table.
- Write all postulates of Dalton's atomic theory. Also mention its importance and limitations.
- Explain the mole concept in detail. What is Avogadro's number? How is molar mass related to atomic mass? Solve: Find moles in 54 g of water and in 4.6 g of sodium.
- Explain ions, their types, and formation. Draw a table of 8 common ions (4 cations, 4 anions). Write 3 chemical formulae using the criss-cross method.
- Write the chemical formulae of the following compounds using the valency/criss-cross method: (a) Aluminium oxide (b) Calcium nitrate (c) Sodium carbonate (d) Ammonium sulphate (e) Magnesium phosphate.
- Calculate the molecular masses of: H₂O, CO₂, HNO₃, CaCO₃, Al₂O₃. Show step-by-step working.
- What is atomic mass? Explain relative atomic mass with reference to Carbon-12. Give the atomic masses of 10 elements.
- Write a comprehensive note on types of molecules: molecules of elements and molecules of compounds. Give 5 examples of each with formulae and molecular mass.
- Explain atomicity with full classification. Why do noble gases exist as monoatomic and nitrogen as diatomic? What is the significance of P₄ and S₈?
- A student takes 9 g of water. Calculate: (a) Moles of water, (b) Molecules of water, (c) Atoms of hydrogen, (d) Atoms of oxygen, (e) Total number of atoms.
✅ MCQ Practice Set — 50 Questions
Basic · Moderate · Exam-Oriented — with answers and explanations
🟢 Basic Level (Q1–20)
Q1. The smallest particle of an element that retains its chemical properties is called:
Answer: B
An atom is the smallest unit of an element retaining chemical identity.
Q2. Avogadro's number is:
Answer: C
Avogadro's number = 6.022 × 10²³ mol⁻¹ (memorize this!)
Q3. The law of constant proportions was stated by:
Answer: C
Q4. The symbol of Potassium is derived from:
Answer: B — K comes from Kalium
Q5. What is the atomicity of ozone (O₃)?
Answer: C — Triatomic
Q6. The molecular mass of water (H₂O) is:
Answer: B — 2(1)+16=18
Q7. Which of the following is a diatomic molecule?
Answer: C
Q8. The formula mass of NaCl is:
Answer: C — 23+35.5=58.5
Q9. The chemical formula of Aluminium Oxide is:
Answer: C — Al(3) and O(2): criss-cross gives Al₂O₃
Q10. How many moles are in 44 g of CO₂?
Answer: B — 44g/44g·mol⁻¹ = 1 mol
Q11. The Latin name of Iron (Fe) is:
Answer: B — Fe comes from Ferrum
Q12. Noble gases are:
Answer: A
Q13. The unit of atomic mass is:
Answer: C
Q14. The symbol 'Au' belongs to which element?
Answer: B — Au from Aurum (Latin for Gold)
Q15. 1 mole of any substance contains how many particles?
Answer: C
Q16. Who proposed Dalton's Atomic Theory?
Answer: C
Q17. The molecular formula of ammonia is:
Answer: B
Q18. If an atom loses electrons, it becomes a:
Answer: A — Losing electrons → positive charge → cation
Q19. The molar mass of oxygen gas (O₂) is:
Answer: C — 2×16=32
Q20. Sulphur exists as:
Answer: D — Polyatomic, S₈
🟡 Moderate Level (Q21–40)
Q21. The number of moles of hydrogen atoms in 18 g of water is:
Answer: B — 1 mole H₂O contains 2 moles of H atoms
Q22. The formula of Calcium Nitrate is:
Answer: B — Ca²⁺ and NO₃⁻ → Ca(NO₃)₂
Q23. Which has maximum number of atoms: 1 mole of H₂, 1 mole of N₂, or 1 mole of He?
Answer: A — H₂ has 2 atoms/molecule, N₂ has 2 atoms/molecule (both = 2Nₐ atoms); He has 1Nₐ atoms.
Q24. The molecular mass of CaCO₃ is:
Answer: B — Ca(40)+C(12)+3O(48) = 100
Q25. According to Dalton's atomic theory, atoms of the same element are:
Answer: A
Q26. How many oxygen atoms are present in 1 mole of CO₂?
Answer: B — Each CO₂ has 2 O atoms, so 1 mole has 2Nₐ oxygen atoms
Q27. The valency of sulphate ion (SO₄²⁻) is:
Answer: B
Q28. The mass of 2 moles of nitrogen gas (N₂) is:
Answer: C — Molar mass N₂ = 28 g/mol × 2 moles = 56 g
Q29. Which statement correctly defines a mole?
Answer: B
Q30. The formula of Aluminium Sulphate is:
Answer: C — Al³⁺ and SO₄²⁻: criss-cross gives Al₂(SO₄)₃
Q31. Phosphorus exists as which molecule?
Answer: C — Tetraatomic
Q32. One atomic mass unit (1 u) equals:
Answer: B
Q33. The correct formula of sodium sulphate is:
Answer: B
Q34. How many grams of hydrogen are there in 1 mole of water?
Answer: B — H₂O has 2H; 2×1=2g hydrogen per mole
Q35. Which of the following is a polyatomic anion?
Answer: C — SO₄²⁻ is polyatomic (multiple atoms) and anionic (negative)
Q36. The law of conservation of mass was given by:
Answer: A — Antoine Lavoisier (1774)
Q37. In a compound, if element A has valency 3 and element B has valency 2, the formula is:
Answer: C — Criss-cross: A(3) B(2) → A₂B₃
Q38. The number of atoms in 0.5 mol of helium gas is:
Answer: B — 0.5 × 6.022×10²³ = 3.011×10²³
Q39. The valency of phosphate ion (PO₄³⁻) is:
Answer: C
Q40. The molar mass of CaCO₃ is:
Answer: B — 40+12+48=100
🔴 Exam-Oriented Level (Q41–50)
Q41. A compound has the formula M₂O₃. The valency of M is:
Answer: C — M₂O₃ → M has valency 3, O has valency 2
Q42. Which is NOT a limitation of Dalton's theory?
Answer: C — Atoms combining in whole numbers is a VALID postulate, not a limitation.
Q43. 3.011 × 10²³ molecules of CO₂ weigh:
Answer: B — Moles = 3.011×10²³ / 6.022×10²³ = 0.5 mol; mass = 0.5×44 = 22 g
Q44. The total number of atoms in H₂SO₄ is:
Answer: C — 2H + 1S + 4O = 7 atoms
Q45. If 4 g of NaOH is dissolved in water, the number of moles is: (Molar mass NaOH = 40)
Answer: C — 4/40 = 0.1 mol
Q46. Which law supports: "In CO₂, carbon and oxygen are always in ratio 3:8"?
Answer: B
Q47. The mass of 6.022 × 10²² molecules of water is:
Answer: B — 6.022×10²² = 0.1 mol; mass = 0.1×18 = 1.8 g
Q48. Which element has the highest atomic mass among: H, C, N, O?
Answer: D
Q49. How many moles of atoms are in 27 g of aluminium? (Al = 27)
Answer: B — 27/27 = 1 mol
Q50. In the formula Ca(OH)₂, what is the total number of atoms?
Answer: C — 1 Ca + 2 O + 2 H = 5 atoms
📅 Previous Year Questions (SEBA / Assam Board)
Frequently asked questions from past board exams
🔁 Recurring Questions — These appear almost every year!
- State the law of conservation of mass and verify it with an example. 3 Marks
- What is Dalton's atomic theory? State its postulates. 3–5 Marks
- What is a mole? What is Avogadro's number? Calculate the number of molecules in 9 g of water. 3 Marks
- Write the chemical formula of: (a) Aluminium oxide (b) Calcium nitrate (c) Sodium carbonate. 2 Marks
- Define atomic mass unit. Write atomic masses of H, O, N, C, Na. 2 Marks
- Define ion. Distinguish between cation and anion with examples. 2 Marks
- Calculate the molecular mass of: H₂O, CO₂, NaCl. 2 Marks
- What is atomicity? Classify: He, H₂, O₃, P₄. 2 Marks
- State the law of constant proportions. Why is water from different sources same in composition? 3 Marks
- Calculate: How many moles and molecules are present in 44 g of CO₂? 3 Marks
- Write the symbols of: Na, K, Fe, Cu, Au. What do these symbols represent? 1–2 Marks
- Give two limitations of Dalton's atomic theory. 1–2 Marks
- What is the difference between molecules of elements and molecules of compounds? 2 Marks
- Why do we use the term 'formula mass' for ionic compounds instead of 'molecular mass'? 2 Marks
- Calculate the number of atoms in 0.5 mol of oxygen gas. 3 Marks
🧠 HOTS — Higher Order Thinking Questions
Assertion-Reason · Case-Based · Competency-Based
🔍 Assertion-Reason Type
Directions: Select A if both assertion and reason are true and reason explains assertion. B if both true but reason doesn't explain. C if assertion true, reason false. D if assertion false.
Q1. Assertion: The mass of reactants equals mass of products in a reaction.
Reason: Atoms are neither created nor destroyed in chemical reactions.
Reason: Atoms are neither created nor destroyed in chemical reactions.
Answer: A — Both true, reason correctly explains assertion (Law of Conservation of Mass).
Q2. Assertion: Noble gases are monoatomic.
Reason: Noble gases have completely filled electron shells and don't react with other atoms.
Reason: Noble gases have completely filled electron shells and don't react with other atoms.
Answer: A — Correct explanation for monoatomic nature.
Q3. Assertion: Dalton's theory said atoms are indivisible.
Reason: This postulate is still completely accepted today.
Reason: This postulate is still completely accepted today.
Answer: C — Assertion is the original postulate (true then); reason is false (atoms ARE divisible — limitation of Dalton's theory).
📊 Case-Based Questions
Case Study: Riya performs an experiment. She takes 10 g of barium chloride and 7.3 g of sodium sulphate in separate containers and mixes them. A white precipitate forms. After the reaction, she finds the product barium sulphate weighs 11.65 g and sodium chloride weighs 5.85 g.
(a) What is the total mass of reactants?
(b) What is the total mass of products?
(c) Which law is verified here?
(d) Write the balanced equation for this reaction.
(a) What is the total mass of reactants?
(b) What is the total mass of products?
(c) Which law is verified here?
(d) Write the balanced equation for this reaction.
(a) Total mass of reactants = 10 + 7.3 = 17.3 g
(b) Total mass of products = 11.65 + 5.85 = 17.3 g
(c) Law of Conservation of Mass is verified.
✅ (d) BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl
💡 Competency-Based Questions
Q1. A scientist finds a new compound with formula X₂Y₃. If X has atomic mass 27 and Y has atomic mass 16, find: (a) Molecular mass (b) Moles in 102 g (c) Number of Y atoms in 0.5 mol
(a) Molecular mass = 2(27) + 3(16) = 54 + 48 = 102 u
(b) Moles = 102/102 = 1 mole [This is Al₂O₃!]
✅ (c) Y atoms in 0.5 mol = 0.5 × 3 × 6.022×10²³ = 9.033 × 10²³ atoms
Q2. Water from the Amazon River and water from a laboratory have different colors and clarity. Yet they have the same chemical properties. Explain using a law from this chapter.
✅ This illustrates the Law of Constant Proportions. Regardless of source, water always contains H and O in 1:8 mass ratio (2:1 atomic ratio). Physical appearance differs, but chemical composition remains constant.
📋 Chapter Summary & Quick Revision
✅ Key Definitions
- Atom: Smallest particle of an element retaining chemical identity
- Molecule: Smallest particle of a substance existing independently
- Mole: Amount of substance = 6.022 × 10²³ particles
- Atomic Mass: Relative mass compared to 1/12 of C-12
- Molecular Mass: Sum of atomic masses of all atoms in a molecule
- Ion: Charged particle formed by gain/loss of electrons
- Cation: Positive ion (loses electrons)
- Anion: Negative ion (gains electrons)
- Atomicity: Number of atoms in one molecule of an element
- Avogadro's Number: 6.022 × 10²³ mol⁻¹
⚡ Last-Minute Revision Sheet
📐 Key Formulae
- n = m/M
- N = n × Nₐ
- Nₐ = 6.022 × 10²³
- 1 u = 1.66 × 10⁻²⁷ kg
⚛️ Atomic Masses (Must Know)
H=1, C=12, N=14, O=16, Na=23, Mg=24, Al=27, P=31, S=32, Cl=35.5, Ca=40, Fe=56, Cu=63.5, Zn=65📜 Two Laws
Lavoisier (1774): Conservation of MassProust (1799): Constant Proportions
Both laws → explained by Dalton's theory
🔬 Dalton's Theory
Key postulates: Atoms indivisible, identical same element, combine in whole numbers, neither created nor destroyed🔢 Atomicity
Mono: He,Ne,ArDi: H₂,O₂,N₂,Cl₂
Tri: O₃, CO₂
Tetra: P₄
Poly: S₈
🏆 Exam Tips
✓ Learn criss-cross method✓ Memorize atomic masses
✓ Practice mole numericals
✓ Know Latin symbol origins
✓ Revise ion valencies
⚠️ Common Mistakes Students Make
Avoid these errors and score full marks!
❌ Mistake 1: Writing Ca₂O₂ for Calcium Oxide
✅ Correct: CaO — Always simplify! Ca(2) O(2) → 1:1 ratio → CaO
❌ Mistake 2: Confusing atomic mass of Cl as 35 instead of 35.5
✅ Correct: Chlorine's atomic mass = 35.5 u (average of isotopes Cl-35 and Cl-37)
❌ Mistake 3: Using 6.022 × 10²² instead of 6.022 × 10²³ for Avogadro's number
✅ Correct: Avogadro's Number = 6.022 × 10²³ — one of the most tested facts!
❌ Mistake 4: Writing AlSO₄ for Aluminium Sulphate
✅ Correct: Al₂(SO₄)₃ — Al has valency 3, SO₄²⁻ has valency 2 → criss-cross gives 2 Al and 3 SO₄
❌ Mistake 5: Confusing molecular mass with molar mass
✅ Correct: Molecular mass is in 'u'; Molar mass is in 'g/mol'. Numerically equal but units differ!
❌ Mistake 6: Forgetting brackets in formulae like Ca(NO₃)₂ → writing CaNO₃₂
✅ Correct: When a polyatomic ion has subscript > 1, always use brackets: Ca(NO₃)₂, Al₂(SO₄)₃
❌ Mistake 7: Mixing up Law of Conservation of Mass (Lavoisier) with Law of Constant Proportions (Proust)
✅ Memory Aid: Lavoisier = Loss (nothing is lost), Proust = Proportions (fixed ratio)
❌ Mistake 8: Saying molecules of elements don't exist (e.g., Hydrogen exists as H, not H₂)
✅ Correct: Hydrogen naturally exists as H₂ (diatomic molecule). Similarly O₂, N₂, Cl₂.
❓ Frequently Asked Questions (15 FAQs)
All common doubts cleared!
Q1. What is an atom?
An atom is the smallest particle of an element that retains the chemical identity of that element. It may or may not exist independently. For example, hydrogen atoms exist as H₂ (paired), while noble gas atoms like He exist alone.
Q2. What is a molecule?
A molecule is the smallest particle of a substance (element or compound) that can exist independently and retains all the physical and chemical properties of that substance. E.g., H₂O (water molecule) has all properties of water.
Q3. What is Avogadro's Number?
Avogadro's Number (Nₐ) = 6.022 × 10²³ mol⁻¹. It represents the number of atoms, molecules, or ions in one mole of any substance. Named after Italian scientist Amedeo Avogadro.
Q4. What is the difference between an atom and a molecule?
An atom is the smallest unit of an element (e.g., O). A molecule is made of two or more atoms bonded together (e.g., O₂). Atoms may not exist freely; molecules always exist independently.
Q5. Why does water always have the same composition?
Due to the Law of Constant Proportions (Proust). Water always has H:O in the ratio 1:8 by mass and 2:1 by number of atoms, regardless of its source.
Q6. What is the mole concept?
The mole is a unit of measurement. One mole = 6.022 × 10²³ particles. It allows chemists to count atoms and molecules by weighing them. 1 mole of any substance has a mass equal to its molar mass in grams.
Q7. Why do some elements have symbols from Latin names?
Early scientists named elements in Latin. Modern symbols often come from these Latin names: Na (Natrium=Sodium), K (Kalium=Potassium), Fe (Ferrum=Iron), Cu (Cuprum=Copper), Au (Aurum=Gold), Pb (Plumbum=Lead).
Q8. What is atomic mass? Why is Carbon-12 used as reference?
Atomic mass is the mass of an atom relative to 1/12 of a Carbon-12 atom. Carbon-12 is used as reference because it is a stable, common isotope, and using 1/12 of its mass gives a convenient scale where most elements have whole-number masses.
Q9. What is the difference between atomic mass and molar mass?
Atomic mass is expressed in u (atomic mass units) for one atom. Molar mass is expressed in g/mol for one mole of atoms. Numerically they are equal (e.g., O has atomic mass 16 u and molar mass 16 g/mol).
Q10. What is the criss-cross method?
The criss-cross method is used to write chemical formulae. Write the valency of each ion, then swap them as subscripts. Example: Mg²⁺ Cl⁻ → MgCl₂ (2 is crossed to Cl, 1 is crossed to Mg). Always simplify to lowest ratio.
Q11. What is the significance of Dalton's atomic theory?
Dalton's theory was the first scientific, evidence-based explanation of atomic structure. It explained the laws of chemical combination, introduced the concept of atomic mass, and laid the foundation for modern chemistry.
Q12. Are Atoms Real? Can We See Them?
Yes, atoms are real and have been imaged using modern tools like Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM). However, they cannot be seen with naked eyes as their radius (~0.1 nm) is billions of times smaller than visible wavelengths.
Q13. What is the difference between molecular mass and formula mass?
Molecular mass applies to covalent compounds (e.g., H₂O, CO₂) that exist as discrete molecules. Formula mass applies to ionic compounds (e.g., NaCl, CaCO₃) that exist as ion lattices, not discrete molecules.
Q14. Why is H₂ diatomic but He monoatomic?
Helium is a noble gas with a completely filled electron shell — it has no need to bond with other atoms. Hydrogen has one electron in its outer shell and readily bonds with another hydrogen atom to achieve a stable configuration, forming H₂.
Q15. How do I remember the formulae of compounds?
Use the criss-cross method: (1) Write symbols side by side, (2) Write valencies below, (3) Swap them as subscripts, (4) Simplify. Practice 5 formulae daily. Also, learning valency tables by heart makes formula writing automatic!
🌟 Conclusion & Motivation
🎉 Congratulations on completing the full study of Class 9 Science Chapter 3 — Atoms and Molecules!
Remember: "Master Atoms and Molecules today, and Chemistry becomes much easier tomorrow." Every formula you learn, every numerical you solve, every law you understand — it all builds towards a deeper understanding of the world around you.
From the air you breathe (N₂ and O₂) to the salt on your table (NaCl) — atoms and molecules are everywhere. Understanding them gives you the power to understand chemistry itself!
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⚛️ Chapter 3: Atoms & Molecules
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