Complete Solutions and Summary of Atoms and Molecules – NCERT Class 9, Science, Chapter 3 – Summary, Questions, Answers, Extra Questions
Detailed summary and explanation of Chapter 3 “Atoms and Molecules” with all question answers, extra questions, and solutions from NCERT Class IX, Science.
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Tags: Atoms and Molecules, Class 9 Science, NCERT, Chapter 3, Dalton’s Atomic Theory, Laws of Chemical Combination, Molecules, Atomic Mass, Molecular Mass, Valency, Chemical Formula, Summary, Answers, Extra Questions
Atoms and Molecules
Chapter 3: Science
Complete Study Guide with Interactive Learning
Chapter Overview
1808
Dalton's Atomic Theory
6.022×10^23
Avogadro's Number
1u
Atomic Mass Unit
2
Laws of Chemical Combination
What You'll Learn
Laws of Chemical Combination
Understanding conservation of mass and constant proportions.
Dalton's Atomic Theory
Postulates explaining atomic nature of matter.
Atoms and Molecules
Symbols, atomic mass, molecular mass, ions.
Mole Concept
Avogadro's number, molar mass, calculations.
Historical Context
The chapter explores ancient ideas from Indian (Kanad, Katyayama) and Greek (Democritus, Leucippus) philosophers on divisibility of matter. Lavoisier and Proust established laws of chemical combination. Dalton's 1808 theory provided explanation. Modern symbols and mole concept by Avogadro.
Key Highlights
Matter consists of atoms, indivisible particles. Laws: mass conserved, elements combine in fixed ratios. Atomic mass unit = 1/12 of C-12. Mole = 6.022×10^23 particles. Chemical formulae, valency for writing formulas.
Comprehensive Chapter Summary
1. Laws of Chemical Combination
Lavoisier established conservation of mass: mass neither created nor destroyed in reactions. Proust's constant proportions: elements in compounds have fixed mass ratios.
2. Dalton's Atomic Theory
Postulates
All matter made of atoms; indivisible; identical for element; combine in whole numbers.
Explanations
Explains conservation (atoms rearranged) and proportions (fixed ratios).
Limitations
Atoms divisible into subatomic particles; isotopes have different masses.
3. Atoms
Size and Mass
Very small, radius in nm; atomic mass in u, relative to C-12.
Symbols
One or two letters, e.g., H for hydrogen, Fe for iron.
Atomic Mass
Average considering isotopes; used for calculations.
4. Molecules and Ions
Molecules
Groups of atoms; atomicity (mono, di, poly); molecular mass sum of atomic masses.
5. Chemical Formulae
Writing Formulae
Using valency, criss-cross method, e.g., NaCl, MgCl2.
Polyatomic Ions
Groups with charge, e.g., SO4^2-, used in formulae.
6. Mole Concept
Mole = 6.022×10^23 particles; molar mass in g/mol. Calculations for mass, number of particles, moles.
Key Concepts and Definitions
Atom
Smallest indivisible particle of element.
Molecule
Group of atoms chemically bonded.
Atomic Mass Unit
1/12 mass of C-12 atom.
Mole
6.022×10^23 entities.
Valency
Combining capacity of element.
Ion
Charged particle, cation or anion.
Important Facts and Figures
1808
Dalton Theory
1u
= 1.66×10^-24 g
6.022×10^23
Avogadro Number
1:8
H:O in Water
14:3
N:H in Ammonia
Questions and Answers from Chapter
Short Questions
Q1. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Answer: Atoms cannot be created nor destroyed.
Q2. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Answer: Relative number and kinds of atoms constant in compound.
Q3. Define the atomic mass unit.
Answer: 1/12 mass of C-12 atom.
Q4. Why is it not possible to see an atom with naked eyes?
Answer: Too small, in nanometers.
Q5. What is meant by the term chemical formula?
Answer: Symbolic representation of composition.
Q6. What is the mass of 1 mole of nitrogen atoms?
Answer: 14 g.
Q7. What is the mass of 4 moles of aluminium atoms?
Answer: 108 g.
Q8. What is the mass of 0.2 mole of oxygen atoms?
Answer: 3.2 g.
Q9. What is the mass of 0.5 mole of water molecules?
Answer: 9 g.
Q10. Give the names of the elements present in quick lime.
Answer: Calcium, oxygen.
Q11. Give the names of the elements present in hydrogen bromide.
Answer: Hydrogen, bromine.
Q12. Give the names of the elements present in baking powder.
Answer: Sodium, hydrogen, carbon, oxygen.
Q13. Give the names of the elements present in potassium sulphate.
Answer: Potassium, sulphur, oxygen.
Q14. What are polyatomic ions?
Answer: Charged groups of atoms.
Q15. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Answer: 1.99×10^-23 g.
Medium Questions
Q1. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Answer: 24 g. Ratio 1:8, so 3:24. (3 marks)
Q2. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?
Answer: 11 g. Law of constant proportions. (3 marks)
Q3. Write down the formulae of sodium oxide.
Answer: Na2O. Valency criss-cross. (3 marks)
Q4. Write down the formulae of aluminium chloride.
Answer: AlCl3. Valency 3 and 1. (3 marks)
Q5. Write down the formulae of sodium sulphide.
Answer: Na2S. Valency 1 and 2. (3 marks)
Q6. Write down the formulae of magnesium hydroxide.
Answer: Mg(OH)2. Polyatomic ion. (3 marks)
Q7. How many atoms are present in a H2S molecule?
Answer: 3 atoms. 2H + 1S. (3 marks)
Q8. How many atoms are present in a PO4^3- ion?
Answer: 5 atoms. 1P + 4O. (3 marks)
Q9. Calculate the molecular masses of H2.
Answer: 2 u. 2×1. (3 marks)
Q10. Calculate the molecular masses of O2.
Answer: 32 u. 2×16. (3 marks)
Q11. Calculate the molecular masses of Cl2.
Answer: 71 u. 2×35.5. (3 marks)
Q12. Calculate the molecular masses of CO2.
Answer: 44 u. 12+32. (3 marks)
Q13. Calculate the molecular masses of CH4.
Answer: 16 u. 12+4. (3 marks)
Q14. Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.
Answer: 3.76×10^22. Calculation with mole. (3 marks)
Q15. Convert into mole: 12 g of oxygen gas.
Answer: 0.375 mole. 12/32. (3 marks)
Long Questions
Q1. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium acetate. Show that these observations are in agreement with the law of conservation of mass.
Answer: Reactants: 5.3 + 6 = 11.3 g. Products: 2.2 + 0.9 + 8.2 = 11.3 g. Masses equal, law holds. Explanation of law and calculation.
Q2. 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 by weight.
Answer: Boron: (0.096/0.24)×100 = 40%. Oxygen: 60%. Step-by-step percentage calculation.
Q3. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
Answer: 6.022×10^20 ions. Molar mass Al2O3=102 g/mol, moles=0.0005, Al ions=2×0.0005×NA.
Q4. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?
Answer: Sodium: 4.35 moles > Iron: 1.79 moles. More atoms in Na. Calculations shown.
Q5. Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.
Answer: H2:2, O2:32, Cl2:71, CO2:44, CH4:16, C2H6:30, C2H4:28, NH3:17, CH3OH:32. Each calculation explained.
Q6. Calculate the molar mass of the following substances: Ethyne C2H2, Sulphur molecule S8, Phosphorus molecule P4, Hydrochloric acid HCl, Nitric acid HNO3.
Answer: C2H2:26, S8:256, P4:124, HCl:36.5, HNO3:63. Step-by-step sums.
Q7. What is the mass of 10 moles of sodium sulphite (Na2SO3)?
Answer: 1260 g. Molar mass 126 g/mol ×10. Calculation.
Q8. Convert into mole: 20 g of water, 22 g of carbon dioxide.
Answer: Water:1.11 mol, CO2:0.5 mol. Divisions explained.
Q9. Write the chemical formulae of the following: Magnesium chloride, Calcium oxide, Copper nitrate, Aluminium chloride, Calcium carbonate.
Answer: MgCl2, CaO, Cu(NO3)2, AlCl3, CaCO3. Valency method for each.
Q10. Give the names of the elements present in the following compounds: Quick lime, Hydrogen bromide, Baking powder, Potassium sulphate.
Answer: Quick lime: Ca O, HBr: H Br, Baking: Na H C O, K2SO4: K S O. Elements listed.
Q11. Write down the names of compounds represented by the following formulae: Al2(SO4)3, CaCl2, K2SO4, KNO3, CaCO3.
Answer: Aluminium sulphate, Calcium chloride, Potassium sulphate, Potassium nitrate, Calcium carbonate. Names given.
Q12. Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.
Answer: ZnO:81 u, Na2O:62 u, K2CO3:138 u. Calculations.
Q13. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Answer: 1.99×10^-23 g. 12 / NA.
Q14. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?
Answer: Na has more atoms. Moles Na=4.35, Fe=1.79; times NA.
Q15. 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 by weight.
Answer: B 40%, O 60%. Percentages calculated.
Interactive Knowledge Quiz
Test your understanding of Atoms and Molecules
Quick Revision Notes
Laws
- Conservation of mass
- Constant proportions
- Multiple proportions
Dalton Theory
- Atoms indivisible
- Identical for element
- Combine whole numbers
Atoms/Molecules
- Symbols 1-2 letters
- Atomic mass u
- Molecular mass sum
Mole
- 6.022×10^23
- Molar mass g/mol
- Calculations
Exam Strategy Tips
- Memorize laws
- Practice calculations
- Understand valency
- Draw symbols
- Revise postulates
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