Last Updated on May 15, 2025 by XAM CONTENT
Understanding the key concepts of Class 9 Science Chapter 3 – Atoms and Molecules is crucial for exam success. To make your revision easier, we have created chapterwise mcq questions with Answers for class 9 Science based on the latest syllabus and exam pattern. It is a part of MCQ Questions for CBSE Class 9 Science Series.
These multiple-choice questions will help you assess your knowledge, improve accuracy, and boost confidence for your exams. Whether you are preparing for school tests, online tests or competitive exams, these Atoms and Molecules MCQs will strengthen your conceptual clarity.
| Chapter | Atoms and Molecules |
| Book | Science Textbook for Class 9 |
| Type of Questions | MCQ Questions |
| Nature of Questions | Competency Based Questions |
| Board | CBSE |
| Class | 9 |
| Subject | Science |
| Useful for | Class 9 Studying Students |
| Answers provided | Yes |
| Difficulty level | Mentioned |
| Important Link | Class 9 Science Chapterwise MCQ Questions |
MCQ Questions on Atoms and Molecules Class 9 Science
MCQs
Q1. Which law is supported when the total mass of reactants equals the total mass of products in a chemical reaction?
(a) Law of Constant Proportions
(b) Law of Multiple Proportions
(c) Law of Conservation of Mass
(d) Law of Conservation of Energy
Show Answer
Answer: (c)
Explanation: The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction.
Q2. Dalton’s atomic theory could explain the Law of Constant Proportions because it proposed that:
(a) Atoms can be divided
(b) Atoms of different elements have identical mass
(c) Atoms combine in fixed ratios
(d) Atoms can be created or destroyed
Show Answer
Answer: (c)
Explanation: Dalton’s theory explained that atoms combine in fixed whole number ratios to form compounds, supporting constant composition.
Q3. What is the atomicity of phosphorus?
(a) Diatomic
(b) Monoatomic
(c) Tetra-atomic
(d) Polyatomic
Show Answer
Answer: (c)
Explanation: Phosphorus exists as P₄ molecules and is thus tetra-atomic.
Q4. Which of the following is a polyatomic ion?
(a) Cl–
(b) NH₄⁺
(c) Na⁺
(d) Mg²⁺
Show Answer
Answer: (b)
Explanation: NH₄⁺ is a polyatomic ion consisting of multiple atoms bonded together carrying a net positive charge.
Q5. Which of the following compounds has a molecular mass of 18 u?
(a) HNO₃
(b) CO₂
(c) H₂O
(d) CH₄
Show Answer
Answer: (c)
Explanation: Water (H₂O): 2×1 + 16 = 18 u.
Q6. Which postulate of Dalton’s atomic theory is invalidated by the discovery of isotopes?
(a) Atoms combine in fixed ratios
(b) Atoms are indivisible
(c) Atoms of same element have same mass
(d) Atoms participate in chemical reactions
Show Answer
Answer: (c)
Explanation: Isotopes are atoms of the same element with different masses, contradicting Dalton’s postulate.
Q7. Why is a chemical formula like Ca(OH)₂ written with brackets around OH?
(a) OH is an atom
(b) It is a group present twice
(c) It is a cation
(d) It is a polyatomic molecule
Show Answer
Answer: (b)
Explanation: Brackets are used to show that the entire hydroxide group is present twice.
Q8. Which element has the symbol derived from its Latin name?
(a) Sodium
(b) Chlorine
(c) Carbon
(d) Oxygen
Show Answer
Answer: (a)
Explanation: Sodium’s symbol is Na, derived from its Latin name ‘Natrium’.
Q9. What is the molecular mass of carbon dioxide (CO₂)?
(a) 32 u
(b) 28 u
(c) 44 u
(d) 30 u
Show Answer
Answer: (c)
Explanation: CO₂ = 12 (C) + 2×16 (O) = 44 u.
Q10. Which of the following statements is true about atoms?
(a) Atoms can always exist freely
(b) Atoms are visible under a simple microscope
(c) Atoms combine to form molecules
(d) Atoms can be divided in chemical reactions
Show Answer
Answer: (c)
Explanation: Atoms combine to form molecules and may not exist independently.
Q11. Which of the following compounds has both ionic and covalent bonds?
(a) H₂O
(b) NaCl
(c) NH₄Cl
(d) CO₂
Show Answer
Answer: (c)
Explanation: NH₄Cl has covalent bonds within NH₄⁺ and ionic bonds between NH₄⁺ and Cl⁻.
Q12. Which of these has the highest number of atoms?
(a) 18 g of H₂O
(b) 44 g of CO₂
(c) 2 mol of CH₄
(d) 1 mol of O₂
Show Answer
Answer: (c)
Explanation: 2 mol of CH₄ contains 2 × (1 C + 4 H) = 10 atoms × Avogadro’s number.
Q13. Which among the following correctly describes the valency of aluminium in Al₂O₃?
(a) 1
(b) 2
(c) 3
(d) 4
Show Answer
Answer: (c)
Explanation: Each aluminium ion has a +3 charge in Al₂O₃, so valency is 3.
Q14. Why is atomic mass considered a relative mass?
(a) It changes with environment
(b) It is calculated relative to carbon-12
(c) It has no real basis
(d) It is variable for each isotope
Show Answer
Answer: (b)
Explanation: Atomic mass is measured relative to 1/12th of the mass of a carbon-12 atom.
Q15. How many atoms are present in one molecule of aluminium sulphate, Al₂(SO₄)₃?
(a) 5
(b) 17
(c) 12
(d) 15
Show Answer
Answer: (d)
Explanation: 2 Al + 3 S + 12 O = 17 atoms in one molecule.
Q16. Which of the following pairs has the same number of atoms?
(a) 18 g of H₂O and 18 g of H₂
(b) 1 mol of O₂ and 1 mol of O₃
(c) 1 mol of CO₂ and 1 mol of SO₂
(d) 32 g of O₂ and 44 g of CO₂
Show Answer
Answer: (c)
Explanation: Both 1 mol CO₂ and 1 mol SO₂ have 3 atoms per molecule, so same total atoms.
Q17. Which molecule contains the highest number of different elements?
(a) CH₄
(b) HNO₃
(c) H₂SO₄
(d) C₂H₅OH
Show Answer
Answer: (c)
Explanation: H₂SO₄ contains 3 different elements: hydrogen, sulfur, and oxygen.
Q18. What is the mass of 1 mole of calcium carbonate (CaCO₃)?
(a) 50 u
(b) 78 u
(c) 100 u
(d) 150 u
Show Answer
Answer: (c)
Explanation: CaCO₃ = 40 (Ca) + 12 (C) + 48 (3×O) = 100 u.
Q19. Why do atoms combine to form compounds?
(a) To reduce mass
(b) To gain stability
(c) To lose identity
(d) To form ions
Show Answer
Answer: (b)
Explanation: Atoms combine to achieve a stable electronic configuration.
Q20. Which of the following is the correct representation of the formula unit mass of Na₂SO₄?
(a) 46 u
(b) 96 u
(c) 142 u
(d) 118 u
Show Answer
Answer: (c)
Explanation: Na₂SO₄ = 2×23 + 32 + 4×16 = 46 + 32 + 64 = 142 u.
We hope the given mcq questions with Answers for Atoms and Molecules Class 9 helps you in your learning.
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Topics from which mcq questions may be asked
- Law of conservation of mass
- Law of constant proportions
- Mole concept
Atoms and molecules form the building blocks of matter – invisible yet powerful.
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Frequently Asked Questions (FAQs) on Atoms and Molecules MCQ Questions
Q1: What are MCQ questions in Class 9 Science?
A1: MCQ (Multiple Choice Questions) in Class 9 Science are objective-type questions where students choose the correct answer from four options. They test conceptual understanding, application, and reasoning skills.
Q2: Are these Class 9 Science MCQs based on the latest CBSE syllabus?
A2: Yes, all MCQ questions provided are strictly based on the latest CBSE Class 9 Science syllabus and follow the current NCERT textbook.
Q3: Are Class 9 Science MCQ questions useful for board exam preparation?
A3: Yes, practicing MCQ questions helps build strong conceptual understanding and improves performance in school tests, Olympiads, and board-level competency assessments.
Q4: Do these MCQs include answers and explanations?
A4: Absolutely. Each MCQ question includes the correct answer along with a detailed explanation to help students understand the concept behind the question.
Q5: Where can I find the best MCQ questions for Class 9 Science?
A5: You can find high-quality, CBSE-based MCQ questions for Class 9 Science on trusted educational websites like xamcontent.com. These include chapterwise, topicwise, and competency-based questions aligned with the latest syllabus
Q6: Are there any resources available online for students to practice mcq questions on class 9 science chapter 3 “Atoms and Molecules“ for CBSE exams?
A6: Yes, several educational websites offer mcq questions for CBSE students preparing for science examinations. We also offer a collection of mcq questions for all classes and subject on our website. Visit our website to access these questions and enhance your learning experience. for more mcq, you can also visit Physics Gurukul Website.
