⚗️ CBSE · Class 9 · Science · Chapter 3

Atoms and
Molecules

Complete chapter resources for CBSE Class 9 Science — topic breakdown, key concepts, sample questions, previous year board questions, and instant AI question paper generation.

4Topics
5–7Board marks
8Sample questions
3PYQ included
Sign Up Free & Generate → See sample questions ↓

Free for independent teachers · No credit card required

Key Concepts — Chapter 3
  • Mole: 1 mol = 6.022 × 10²³ particles (Nₐ)
  • Molar mass: mass of 1 mol of substance (g/mol)
  • No. of moles: n = given mass / molar mass
  • Law of const. prop.: fixed mass ratio in a compound
  • Atomicity: no. of atoms per molecule of element
  • Formula unit mass: sum of atomic masses in formula
Chapter Overview

What this chapter covers

Chapter 3 introduces the building blocks of matter. An atom is the smallest particle of an element that retains its chemical identity, while a molecule is the smallest particle of a substance — element or compound — that can exist independently. The chapter opens with the two foundational laws of chemical combination: the Law of Conservation of Mass (Lavoisier) and the Law of Constant Proportions (Proust), both of which govern every chemical reaction students will encounter through their chemistry education.

Dalton's Atomic Theory provides a particle-level model that explains these laws, and the chapter uses it to define atomic mass (measured in atomic mass units, u) and molecular mass. Students learn to write and interpret chemical formulas using valency — the combining capacity of an atom — and to calculate formula unit mass for ionic compounds. Atomicity (the number of atoms in one molecule of an element) distinguishes monoatomic (Na, He), diatomic (H₂, O₂, N₂, Cl₂), and polyatomic (P₄, S₈) elements.

The final and most calculation-intensive topic is the mole concept: 1 mole of any substance contains Avogadro's number (6.022 × 10²³) of particles and has a mass in grams equal to its molar mass. Mole concept questions — converting between mass, moles, and number of particles — appear consistently in board exams and account for the highest marks in this chapter.

Topics

What's inside Chapter 3

As per NCERT Class 9 Science (CBSE syllabus)

Topic 1
Laws of Chemical Combination
Law of Conservation of Mass: total mass of reactants = total mass of products. Law of Constant Proportions: elements in a compound always combine in a fixed mass ratio, regardless of source.
Topic 2
Atoms — Symbol, Size, and Atomic Mass
Dalton's atomic theory, atomic symbols (IUPAC), relative atomic mass on the carbon-12 scale (u), and atomicity of common elemental molecules (monoatomic, diatomic, polyatomic).
Topic 3
Molecules and Chemical Formulas
Molecular mass as the sum of atomic masses. Writing chemical formulas using valency and the criss-cross rule. Formula unit mass of ionic compounds (e.g., NaCl, CaCO₃). Common polyatomic ions.
Topic 4
Mole Concept and Molar Mass
1 mole = 6.022 × 10²³ particles (Avogadro's number). Molar mass in g/mol. Calculations: n = mass / molar mass, and number of particles = n × Nₐ. Converting between mass, moles, and particle count.
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Ch 1 · Matter in Our Surroundings
Particle nature of matter, states, and physical properties
Ch 2 · Is Matter Around Us Pure?
Elements, compounds, and mixtures — the classification of matter
Chapter 3 Atoms &
Molecules
Leads to
Ch 4 · Structure of the Atom
Sub-atomic particles, Bohr model, electronic configuration
Class 10 · Chemical Reactions
Balancing equations, types of reactions — directly requires mole concept
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 9 Science examination papers from the last five years.

Question type Marks Typical count What's usually tested
MCQ / Objective 1 1–2 Atomicity, chemical symbol, or Avogadro's number identification
Very Short Answer 2 1 State a law, write a chemical formula, or define mole
Short Answer 3 1 Calculate molecular mass, moles from given mass, or formula unit mass
Long Answer / Calculation 4–5 1 Multi-step mole concept problem — mass → moles → number of particles
Total (approximate) 5–7 4–5 Weightage varies across paper sets and years
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 9 Science Chapter 3. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
The atomicity of phosphorus (P₄) is: (a) 1 (b) 2 (c) 4 (d) 8
Q2 Easy 2 marks Short Answer
State the Law of Conservation of Mass. How does it apply to the reaction between hydrogen and oxygen to form water?
Q3 Medium 2 marks Short Answer
Write the chemical formula of the following compounds using the criss-cross rule of valency: (i) Magnesium chloride (ii) Aluminium oxide
Q4 Medium 3 marks Short Answer
Calculate the molecular mass of the following: (i) H₂SO₄ (ii) Ca(OH)₂ (iii) HNO₃ (Atomic masses: H = 1, O = 16, S = 32, Ca = 40, N = 14)
Q5 Medium 3 marks Word Problem
Calculate the number of moles in: (i) 46 g of sodium (Na) (ii) 8 g of oxygen gas (O₂) (iii) 22 g of carbon dioxide (CO₂) (Atomic masses: Na = 23, O = 16, C = 12)
Q6 Hard 4 marks Word Problem
A sample of ammonia (NH₃) has a mass of 34 g. (i) Calculate the number of moles of NH₃. (ii) Calculate the number of molecules of NH₃. (iii) Calculate the number of hydrogen atoms present. (iv) What is the mass of nitrogen in this sample? (Atomic masses: N = 14, H = 1; Nₐ = 6.022 × 10²³)
Q7 Hard 4 marks Word Problem
12 g of magnesium burns completely in oxygen to form magnesium oxide (MgO). (i) Write the balanced chemical equation. (ii) Using the Law of Conservation of Mass, find the mass of oxygen consumed. (iii) Calculate the number of moles of MgO formed. (iv) How many formula units of MgO are produced? (Atomic masses: Mg = 24, O = 16)
Q8 Hard 5 marks Case-Based
In an experiment, 18 g of water is electrolysed completely into hydrogen and oxygen gases. (i) State the Law of Constant Proportions and verify it for water (H : O mass ratio = 1 : 8). (ii) Calculate the mass of hydrogen and oxygen produced. (iii) Find the number of moles of each gas formed. (iv) Calculate the number of molecules of hydrogen gas obtained. (v) Name the scientist who proposed this law. (Atomic masses: H = 1, O = 16)
Generate a full paper with answer key →

MarksZen AI creates a complete question paper with answer key in under 2 minutes.

Previous Year Questions

From CBSE board examinations

Actual questions from past Class 9 Science examination papers — Atoms and Molecules chapter.

Board 20223 marks
Calculate the number of particles in each of the following: (i) 46 u of He atom (ii) 8 g of O₂ molecules (iii) 0.1 mol of carbon atoms. (CBSE 2022)
Board 20232 marks
Write the chemical formulae of the following: (i) Calcium phosphate (ii) Aluminium sulphate. Also state the valency of each combining element used. (CBSE 2023)
Board 20202 marks
State the Law of Constant Proportions. Verify it using the example of water, given that water always contains hydrogen and oxygen in the mass ratio 1 : 8. (CBSE 2020)
For Teachers

Create a board-aligned
question paper in 2 minutes.

Pick chapter, set the question-type mix and total marks — MarksZen AI generates the full paper with answer key. CBSE, ICSE, and all State Boards supported.

  • All 4 topics of this chapter
  • MCQ + short answer + word problems
  • Answer key included
  • PDF export ready
Sign Up Free & Generate →
FAQ

Questions teachers ask

How many marks does Atoms and Molecules carry in the CBSE Class 9 Science exam? +
Atoms and Molecules typically carries 5–7 marks in the CBSE Class 9 Science annual examination. Questions usually include one 1-mark objective question on atomic mass or chemical formula, one 2-mark short answer on laws of chemical combination, and one 3-mark question on mole concept calculations. The exact weightage varies by school and paper set.
What is Avogadro's number and how is it used in board exam questions? +
Avogadro's number (Nₐ) is 6.022 × 10²³ — the number of particles (atoms, molecules, or ions) present in one mole of any substance. Board questions commonly ask students to calculate the number of atoms in a given mass of an element, or the number of molecules in a given number of moles. Always write the value as 6.022 × 10²³ mol⁻¹ for full marks.
What is the difference between atomicity and valency? +
Atomicity is the number of atoms present in one molecule of an element — for example, O₂ has atomicity 2 (diatomic), and P₄ has atomicity 4. Valency is the combining capacity of an atom, determined by the number of electrons it gains, loses, or shares to complete its outermost shell. Atomicity is a property of an elemental molecule; valency is a property of an individual atom used to write chemical formulas.
How do I write the chemical formula of a compound for board exams? +
To write a chemical formula, list the symbols of the combining elements, write their valencies below each symbol, and then criss-cross the valency numbers as subscripts (reducing to simplest whole-number ratio if needed). For example, calcium (valency 2) and chlorine (valency 1) gives CaCl₂. Always check that the overall charge of the formula unit is zero.
How do I generate a custom question paper for Atoms and Molecules using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 9 Science, select Chapter 3 (Atoms and Molecules), set your preferred question-type mix (MCQ, short answer, word problem) and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.