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I. Chapter Summary:

This chapter introduces the basic concepts of atoms and molecules, forming the foundation of chemistry. Students explore how all matter is composed of atoms and molecules, the laws of chemical combination, and how atoms combine to form molecules and compounds. The chapter explains atomic mass, molecular mass, valency, chemical formulae, and the concept of the mole. Learners also gain the ability to perform calculations based on moles, mass, and number of particles using Avogadro’s number.

II. Key Concepts Covered:

Concept	Description
Laws of Chemical Combination	Law of conservation of mass and law of constant proportions.
Atom	Smallest particle of an element that may or may not have an independent existence.
Molecule	Group of atoms chemically bonded; can be of elements (O₂) or compounds (H₂O).
Atomic Mass	Mass of an atom expressed in atomic mass units (u or amu).
Molecular Mass	Sum of atomic masses in a molecule.
Valency	Combining capacity of an atom or ion.
Chemical Formulae	Representation of molecules using symbols and valencies.
Mole Concept	A mole represents 6.022 × 10²³ particles (Avogadro number).
Molar Mass	Mass of 1 mole of a substance in grams.

III. Important Questions:

(A) Multiple Choice Questions (1 Mark):

1. Which of the following represents a diatomic molecule?
   a) Na
   b) O₂ ✅
   c) Cl
   d) C
2. The SI unit of the amount of substance is:
   a) Kilogram
   b) Mole ✅
   c) Gram
   d) Molecule
3. What is the valency of carbon in methane (CH₄)?
   a) 1
   b) 2
   c) 3
   d) 4 ✅
4. What is the molecular mass of CO₂? (Atomic masses: C = 12 u, O = 16 u)
   a) 28 u
   b) 32 u
   c) 44 u ✅
   d) 30 u

(B) Short Answer Questions (2/3 Marks):

1. State the law of constant proportions with an example.
2. Define molecular mass. How is it calculated?
3. Write the chemical formula of:
   a) Sodium oxide
   b) Aluminium chloride
4. Calculate the number of molecules in 18 g of water.
   – PYQ 2021

(C) Long Answer Questions (5 Marks):

1. Define a mole. Calculate the number of molecules in 36 g of water.
2. Explain the steps to write a chemical formula. Write the formulae for magnesium nitrate and calcium oxide.
3. What are the two laws of chemical combination? Explain them with suitable examples.
4. Calculate:
   a) The molar mass of Ca(OH)₂
   b) The number of atoms in 16 g of oxygen gas (O₂)
   – PYQ 2022

(D) HOTS (Higher Order Thinking Skills):

1. Why do noble gases like helium and neon have a valency of zero?
2. You have 12 g of carbon and 32 g of oxygen. What is the ratio by mass in CO₂? Does it follow the law of constant proportions?

IV. Key Formulas/Concepts:

Concept/Formula	Explanation
1 mole =	6.022 × 10²³ particles (atoms/molecules/ions)
Molar mass	= Atomic mass or molecular mass in grams
Number of moles	= Given mass / Molar mass
Number of particles	= Moles × Avogadro’s number
Chemical formula	Based on valency: Criss-cross method
Atomic mass of H, C, O, N, Cl, Na, Mg, etc.	H = 1 u, C = 12 u, O = 16 u, N = 14 u, Cl = 35.5 u, Na = 23 u, Mg = 24 u

V. Deleted Portions (CBSE 2025–2026):

 No portions have been deleted from this chapter as per the rationalized NCERT textbooks.

VI. Chapter-Wise Marks Bifurcation (Estimated – CBSE 2025–2026):

Unit/Chapter	Estimated Marks	Type of Questions Typically Asked
Chapter 3: Atoms and Molecules	8–10 marks	Numerical problems, definitions, chemical formula writing

VII. Previous Year Questions (PYQs):

Year	Marks	Question
2020	3	Write the molecular formulae of ammonium chloride and sodium carbonate.
2021	5	Calculate the number of particles in 4.4 g of CO₂.
2022	2	State the law of conservation of mass with an activity-based explanation.

VIII. Real-World Application Examples to Connect with Topics:

Concept	Real-World Application
Chemical Formulae	Used in pharmaceuticals to write drug compositions.
Law of Constant Proportions	Ensures consistency in chemical manufacturing.
Mole Concept	Used in quantitative analysis of chemical reactions.
Avogadro’s Number	Applied in stoichiometric calculations in chemistry labs.

IX. Student Tips & Strategies for Success:

 Time Management:

• Allocate 20 minutes per session for concepts + 10 minutes for numerical problems.
• Make a formula sheet for quick revision.

 Exam Preparation:

• Focus on chemical formula writing, mole numericals, and laws of combination.
• Practice all NCERT exemplar and intext numericals.
• Highlight all atomic masses in the textbook.

 Stress Management:

• Use mnemonics to remember valencies.
• Draw concept maps for stepwise formula writing and mole calculations.

X. Career Guidance & Exploration (Class 9–10 Specific):

Stream	Career Paths
Science	Chemist, Chemical Engineer, Pharmacist
Medical	Pathologist, Biotechnologist, Toxicologist
Applied Fields	Food Analyst, Materials Scientist, Laboratory Technician

• Explore Through:
  • NTSE
  • NSO & IESO
  • School science clubs or molecule modeling activities
  • CBSE Science Exhibition

XI. Important Notes:

•  Do not skip numericals and formula writing—they often carry high weightage.
•  Memorize atomic masses and valencies of common elements.
•  Clearly distinguish between atoms, molecules, ions, and moles in answers.
•  Practice unit conversions and mole–mass–particle relationships thoroughly.