Class 9 Chemistry - Atoms and Molecules | Willer Academy

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Class 9 Chemistry - NCERT Supplement

Chapter 3: Atoms and Molecules

Comprehensive Notes and Practice Test

Chapter 3: Atoms and Molecules - Detailed Notes

1. Laws of Chemical Combination

Chemical reactions follow certain fundamental laws:

Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction.

Law of Constant Proportions: A chemical compound always contains the same elements combined together in the same proportion by mass.

2. Dalton's Atomic Theory

John Dalton proposed his atomic theory in 1808:

All matter is made of tiny indivisible particles called atoms
Atoms of a given element are identical in mass and properties
Compounds are formed by combination of atoms in simple ratios
Chemical reactions involve rearrangement of atoms

Definition: An atom is the smallest particle of an element that can exist and may or may not have independent existence.

3. Atoms and Molecules

Atom

Smallest unit of matter

Example: Na (Sodium)

Molecule

Group of atoms bonded together

Example: H₂O (Water)

Ion

Charged atom or molecule

Example: Na⁺ (Sodium ion)

4. Writing Chemical Formulae

Chemical formulae represent the composition of molecules:

H₂O = 2 Hydrogen atoms + 1 Oxygen atom
CO₂ = 1 Carbon atom + 2 Oxygen atoms
NaCl = 1 Sodium atom + 1 Chlorine atom

5. Molecular Mass and Mole Concept

Molecular Mass: Sum of atomic masses of all atoms in a molecule

Mole Concept: One mole is the amount of substance that contains as many particles (atoms, molecules, ions) as there are atoms in exactly 12g of carbon-12

Example: Molecular mass of water (H₂O) = (2×1) + 16 = 18u. Thus, 1 mole of water = 18g

Element	Symbol	Atomic Mass (u)
Hydrogen	H	1
Carbon	C	12
Oxygen	O	16
Sodium	Na	23

Important: Avogadro's number (6.022 × 10²³) is the number of particles in one mole of any substance.

Multiple Choice Questions (20 Questions)

Select the correct answer for each question. Each question carries 1 mark.

Test Progress: 0/20 questions answered

Short Answer Questions (5 Questions - 2 Marks Each)

Answer these questions in 30-50 words.

1. State the Law of Conservation of Mass. Give an example.

2. Define atomic mass unit. What is the atomic mass of oxygen?

3. What is the difference between an atom and a molecule?

4. Write the chemical formula for magnesium chloride.

5. Calculate the molecular mass of carbon dioxide (CO₂).

Subjective Questions (5 Questions - 5 Marks Each)

Answer these questions in detail (100-150 words). After submitting, model answers are shown for self-evaluation.

1. State and explain Dalton's atomic theory. What are its limitations?

2. Explain the mole concept with examples. How is it related to Avogadro's number?

3. Calculate the number of moles in 180g of water. How many molecules are present?

4. What are ions? Differentiate between cations and anions with examples.

5. Write the chemical formulae for the following compounds: (a) Calcium carbonate (b) Ammonium sulfate (c) Sodium hydroxide

Performance Feedback & Ranking

MCQ Test

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Multiple Choice

Short Answers

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5 Questions

Subjective

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5 Questions

Total Score

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Overall

Overall Performance Summary

You have completed the Chapter 3: Atoms and Molecules test. Here's your performance analysis:

Section	Questions	Score
MCQ Test	20	0/20
Short Answer	5	0/10
Subjective	5	0/25
Total	30	0/55

Strengths:

Excellent understanding of atomic structure and molecules
Strong grasp of mole concept calculations
Accurate chemical formula writing skills

Areas for Improvement:

Need more practice with polyatomic ions
Review limitations of Dalton's atomic theory
Work on molecular mass calculations for complex compounds

Recommendations:

Practice more numerical problems on mole concept. Review the rules for writing chemical formulas. Focus on understanding the significance of Avogadro's number in chemical calculations.

Your Performance Rank

Good effort! With a bit more practice, you can reach the top ranks.

Focus on the improvement areas to achieve better results.