Short Intro
This chapter introduces the atomic nature of matter and explains how atoms combine to form molecules and compounds. Students learn important laws of chemistry, chemical bonding, writing chemical formulae, ionic and covalent compounds, and molecular mass through solved examples and numerical problems.
Quick Information Box
| Topic | Details |
|---|---|
| Chapter | Atomic Foundations of Matter |
| Class | Grade 9 |
| Subject | Science |
| Main Topics | Chemical Bonding, Atoms, Molecules |
| Important Laws | Conservation of Mass, Constant Proportions |
| Key Bond Types | Ionic Bond and Covalent Bond |
Concepts Used (Topics Covered)
- Law of Conservation of Mass
- Law of Constant Proportions
- Dalton’s Atomic Theory
- Molecules and Chemical Bonds
- Covalent Bond Formation
- Ionic Bond Formation
- Cations and Anions
- Writing Chemical Formulae
- Properties of Ionic and Covalent Compounds
- Molecular Mass
- Formula Unit Mass
Important Formulas
- Mass of Reactants = Mass of Products
- Number of Neutrons
= Mass Number − Atomic Number - Molecular Mass
= Sum of Atomic Masses of All Atoms - Formula Unit Mass
= Sum of Atomic Masses in Ionic Compound - Chemical Formula by Criss-cross Method
Questions & Step-by-step Solutions with Explanation
Q1. Law of Conservation of Mass
Question:
A student burns ethanol in an open beaker and no residue remains. Is the law violated?
Solution:
No. During burning, gases like carbon dioxide and water vapour escape into air. Total mass remains conserved when gases are included.
Answer:
Law of Conservation of Mass is not violated.
Q2. Water Formation Problem
Question:
20 g hydrogen reacts completely with 160 g oxygen. Find mass of water formed.
Solution:
According to Law of Conservation of Mass:
Mass of reactants = Mass of products
= 20 + 160
= 180 g
Answer:
180 g water is formed.
Q3. Sodium Chloride Ratio Problem
Question:
NaCl contains sodium and chlorine in ratio 23 : 35.5. If 46 g sodium reacts, how much chlorine is required?
Solution:
23 g sodium reacts with 35.5 g chlorine
46 g sodium reacts with:
= (35.5 × 46) / 23
= 71 g
Answer:
71 g chlorine is needed.
Q4. Sulfur and Oxygen Compound
Question:
Compound contains 40% sulfur and 60% oxygen. If sulfur = 20 g, find oxygen mass.
Solution:
Sulfur : Oxygen = 40 : 60
20 : x = 40 : 60
x = (20 × 60)/40
= 30 g
Answer:
30 g oxygen
Q5. Carbon Monoxide Formation
Question:
CO contains carbon and oxygen in ratio 3 : 4. Find oxygen needed for 9 g carbon.
Solution:
3 g carbon combines with 4 g oxygen
9 g carbon combines with:
= (4 × 9)/3
= 12 g
Answer:
12 g oxygen
Q6. Dalton’s Atomic Theory Assertion
Assertion:
2 g hydrogen combines with 16 g oxygen to form 18 g water.
Reason:
Atoms combine in simple whole number ratios.
Answer:
Both are true and Reason correctly explains Assertion.
Q7. Formation of Hydrogen Molecule
Solution:
Each hydrogen atom has 1 electron.
Two hydrogen atoms share one electron each to complete duplet.
H — H
Bond Type:
Single Covalent Bond
Q8. Formation of Chlorine Molecule
Solution:
Each chlorine atom has 7 valence electrons.
They share one electron pair to complete octet.
Cl — Cl
Bond Type:
Single Covalent Bond
Q9. Formation of Oxygen Molecule
Solution:
Oxygen has 6 valence electrons.
Each oxygen atom shares 2 electrons.
O = O
Bond Type:
Double Covalent Bond
Q10. Formation of Water Molecule
Solution:
Oxygen needs 2 electrons.
Two hydrogen atoms share one electron each with oxygen.
Structure:
H — O — H
Formula:
H₂O
Q11. Why Neon Does Not React
Solution:
Neon has complete octet configuration.
Therefore it is chemically stable and neither gains nor loses electrons.
Q12. Formation of Sodium Chloride
Solution:
- Sodium loses one electron → Na⁺
- Chlorine gains one electron → Cl⁻
Electrostatic attraction forms ionic bond.
Formula:
NaCl
Q13. Oxygen Ion Formation
Solution:
Oxygen gains two electrons.
Ion Formed:
O²⁻
Type:
Anion
Q14. Magnesium Chloride Formation
Solution:
Magnesium loses two electrons → Mg²⁺
Each chlorine gains one electron → Cl⁻
One Mg²⁺ combines with two Cl⁻ ions.
Formula:
MgCl₂
Q15. Sodium Sulfide Formation
Solution:
Two sodium atoms donate one electron each.
Sulfur gains two electrons.
Formula:
Na₂S
Q16. Name the Following
| Compound | Name |
|---|---|
| CO₂ | Carbon dioxide |
| NO₂ | Nitrogen dioxide |
| SF₆ | Sulfur hexafluoride |
| PCl₃ | Phosphorus trichloride |
Q17. Write Formulae
| Compound | Formula |
|---|---|
| Sodium hydrogencarbonate | NaHCO₃ |
| Sulfur dioxide | SO₂ |
| Ferric chloride | FeCl₃ |
| Cuprous oxide | Cu₂O |
Q18. Formula from Ions
(i) Fe³⁺ and OH⁻
Formula:
Fe(OH)₃
(ii) K⁺ and CO₃²⁻
Formula:
K₂CO₃
Q19. Electrical Conductivity Question
Solution:
A compound conducting electricity only in solution is ionic.
Answer:
Ionic bond is present.
Q20. Molecular Mass of Nitric Acid
Formula:
HNO₃
Calculation:
= 1 + 14 + (16 × 3)
= 63 u
Answer:
63 u
Q21. Molecular Mass of Methane
Formula:
CH₄
Calculation:
= 12 + (1 × 4)
= 16 u
Answer:
16 u
Q22. Formula Unit Mass of KCl
Calculation:
= 39 + 35.5
= 74.5 u
Answer:
74.5 u
Q23. Formula Unit Mass of Mg(OH)₂
Calculation:
= 24 + 2(16 + 1)
= 24 + 34
= 58 u
Answer:
58 u
Q24. Electronic Configuration Problem
Given:
11 protons, 12 neutrons, 10 electrons
(i) Atomic Number:
11
Mass Number:
23
(ii) Nature:
Cation because protons > electrons
(iii) Electronic Configuration:
2, 8
(iv) Species:
Na⁺
Common Mistakes
- Confusing ionic and covalent bonds
- Writing wrong electronic configurations
- Forgetting brackets in polyatomic ions
- Ignoring charge balancing while writing formulae
- Incorrect molecular mass calculations
Exam Tips
- Learn valencies of common ions.
- Practice criss-cross method regularly.
- Remember definitions of cation and anion.
- Draw bonding diagrams neatly.
- Revise important laws carefully.
Practice MCQs
1. Which bond is formed by sharing electrons?
A. Ionic Bond
B. Covalent Bond
C. Metallic Bond
D. Hydrogen Bond
Answer:
B. Covalent Bond
2. Which particle is positively charged?
A. Electron
B. Neutron
C. Proton
D. Atom
Answer:
C. Proton
3. Formula of magnesium oxide:
A. Mg₂O
B. MgO₂
C. MgO
D. Mg₂O₂
Answer:
C. MgO
4. Compound conducting electricity in aqueous solution is generally:
A. Covalent
B. Ionic
C. Organic
D. Neutral
Answer:
B. Ionic
5. Molecular mass of water:
A. 16 u
B. 17 u
C. 18 u
D. 20 u
Answer:
C. 18 u
FAQ Section
Q1. What is a covalent bond?
A bond formed by sharing electrons between atoms.
Q2. What is an ionic bond?
A bond formed due to transfer of electrons and electrostatic attraction between ions.
Q3. Why do ionic compounds conduct electricity in water?
Because ions become free to move in solution.
Q4. What is molecular mass?
Sum of atomic masses of all atoms in a molecule.
Q5. What is formula unit mass?
Mass of simplest ratio of ions in an ionic compound.
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