Class 9th Science Atoms & Molecules Important QAs

🔹 Very Short Answer Questions (1 mark)

Q1. Who was the first to use the term ‘atom’?
Ans. The term ‘atom’ was first used by the Greek philosopher Democritus.

Q2. Who gave the modern atomic theory?
Ans. John Dalton.

Q3. What is the smallest particle of an element that takes part in a chemical reaction?
Ans. Atom.

Q4. Define molecule.
Ans. A molecule is the smallest particle of an element or compound that can exist independently and shows all the properties of that substance.

Q5. What is a chemical formula?
Ans. A chemical formula is a symbolic representation of a compound using symbols of elements.

Q6. Write the chemical formula of water and carbon dioxide.
Ans. Water → H₂O; Carbon dioxide → CO₂.

Q7. What is the valency of oxygen?
Ans. 2.

Q8. What is an ion?
Ans. An ion is a charged particle formed by the loss or gain of electrons.

Q9. What are polyatomic ions?
Ans. Ions made up of a group of atoms carrying charge are called polyatomic ions. Example: SO₄²⁻.

Q10. State the law of conservation of mass.
Ans. Mass can neither be created nor destroyed in a chemical reaction.

🔹 Short Answer Questions (2–3 marks)

Q11. State the postulates of Dalton’s Atomic Theory.
Ans.

1. Matter consists of indivisible particles called atoms.
2. Atoms of the same element are identical.
3. Atoms of different elements differ.
4. Atoms combine in whole numbers to form compounds.
5. Atoms cannot be created or destroyed.

Q12. What are the limitations of Dalton’s Atomic Theory?
Ans.

1. It did not explain isotopes and isobars.
2. Atoms are divisible into subatomic particles.
3. It failed to explain the nature of bonding.

Q13. Define atomic mass.
Ans. The total mass of protons and neutrons present in an atom is called its atomic mass.

Q14. What is the difference between atomic mass and molecular mass?
Ans.

• Atomic mass: Mass of one atom of an element.
• Molecular mass: Sum of the atomic masses of all atoms present in a molecule.

Q15. Write two examples of triatomic and polyatomic molecules.
Ans.

• Triatomic: O₃, H₂O
• Polyatomic: P₄, S₈

Q16. What are radicals? Give examples.
Ans. Atoms or group of atoms that carry charge and act as a single unit in reactions. Example: OH⁻, NO₃⁻.

Q17. What is meant by the term formula unit mass?
Ans. The sum of atomic masses of all atoms in a formula unit of an ionic compound.

Q18. Differentiate between a molecule of element and a molecule of compound.
Ans.

• Molecule of element: Made up of atoms of the same element (e.g., O₂, N₂).
• Molecule of compound: Made up of atoms of different elements (e.g., CO₂, H₂O).

Q19. Write the chemical formula of: (i) Sodium chloride (ii) Magnesium oxide (iii) Calcium hydroxide.
Ans. (i) NaCl (ii) MgO (iii) Ca(OH)₂.

Q20. What is the difference between cation and anion?
Ans.

• Cation: Positively charged ion (Na⁺, Ca²⁺).
• Anion: Negatively charged ion (Cl⁻, SO₄²⁻).

🔹 Long Answer Questions (4–5 marks)

Q21. Explain the law of constant proportions with an example.
Ans. According to this law, in a chemical substance, the elements are always present in definite proportions by mass. Example: In water (H₂O), hydrogen and oxygen are always present in the ratio 1:8 by mass.

Q22. Differentiate between atom and molecule.

Q23. Explain the steps involved in writing the chemical formula of a compound with an example.
Ans.

1. Write symbols of elements.
2. Write valencies.
3. Cross the valencies to balance charges.
4. Write the formula.
   Example: For Aluminium oxide → Al³⁺ and O²⁻ → Al₂O₃.

Q24. Write any 5 polyatomic ions with their symbols and charges.
Ans.

• Hydroxide → OH⁻
• Ammonium → NH₄⁺
• Carbonate → CO₃²⁻
• Sulphate → SO₄²⁻
• Nitrate → NO₃⁻

Q25. Explain isotopes and isobars with examples.
Ans.

• Isotopes: Atoms of the same element with same atomic number but different mass number (e.g., ¹²C and ¹⁴C).
• Isobars: Atoms of different elements with same mass number but different atomic number (e.g., ⁴⁰Ca and ⁴⁰Ar).

Q26. How do atoms exist? Give examples.
Ans.

• Atoms of most elements combine to form molecules (H₂, O₂).
• Some exist as monoatomic (He, Ne).
• Some exist as giant structures (C in diamond).

Q27. What are the rules for writing chemical names according to IUPAC?
Ans.

1. Write name of metal (cation) first.
2. Write name of non-metal (anion) with suffix –ide.
3. If polyatomic ion present, use its name as such.
   Example: NaCl → Sodium chloride.

Q28. Explain how laws of chemical combination led to the idea of atoms.
Ans.

• Law of conservation of mass showed atoms are indestructible.
• Law of constant proportions showed atoms combine in fixed ratios.
• Dalton explained both using his atomic theory, proving existence of atoms.

Q29. Give four characteristics of molecules.
Ans.

1. Molecules are formed by atoms.
2. They may be homoatomic or heteroatomic.
3. They exist freely.
4. They represent the chemical properties of the substance.

Q30. Distinguish between monoatomic, diatomic and polyatomic molecules with examples.
Ans.

• Monoatomic → Ar, He
• Diatomic → O₂, H₂
• Polyatomic → O₃, P₄, S₈

🔹 Case Study / Application-Based (5–6 marks)

Q31. Case Study – Hydrogen Peroxide (H₂O₂)
Hydrogen peroxide is a pale blue liquid with chemical formula H₂O₂. It decomposes to give water and oxygen. Answer the following:
(i) What is the molecular mass of H₂O₂?
(ii) Which law of chemical combination is illustrated when H₂O₂ decomposes?
(iii) Name the elements present.
Ans.
(i) H₂O₂ = (2×1) + (2×16) = 34 u
(ii) Law of conservation of mass.
(iii) Hydrogen and Oxygen.

Q32. Case Study – Carbon Dioxide (CO₂)
Carbon dioxide is produced when carbon is burnt in oxygen.
(i) Write the chemical formula.
(ii) State the proportion of C and O by mass.
(iii) Which law is followed here?
Ans.
(i) CO₂.
(ii) C:O = 12:32 = 3:8.
(iii) Law of constant proportions.

Q33. Why are symbols and formulae of elements/compounds important in chemistry?
Ans.

• Symbols represent atoms easily.
• Formulae represent compounds and their composition.
• They help in writing equations and saving time.

Q34. A student wrote formula of calcium chloride as CaCl. Is it correct? Justify.
Ans. No, it should be CaCl₂ because Ca has valency 2 and Cl has valency 1. Cross multiplication gives CaCl₂.

Q35. Why is atomic mass taken as relative mass?
Ans. Because atoms are extremely small, their actual masses are negligible. Hence, atomic mass is expressed relative to carbon-12 isotope.

Q36. Case Study – Ammonium Nitrate (NH₄NO₃)
Ammonium nitrate is used as fertilizer.
(i) Write its formula.
(ii) Name cation and anion.
(iii) Is it a molecular or ionic compound?
Ans.
(i) NH₄NO₃.
(ii) Cation → NH₄⁺; Anion → NO₃⁻.
(iii) Ionic compound.

Q37. Why do isotopes of an element show similar chemical properties?
Ans. Because chemical properties depend on atomic number (electrons), and isotopes have the same atomic number.

Q38. Which postulate of Dalton’s Atomic Theory is not true for isotopes?
Ans. “Atoms of the same element are identical in all respects including mass.”

Q39. Why is the molecular formula of glucose written as C₆H₁₂O₆?
Ans. Because one molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.

Q40. Why is a water molecule said to be polar in nature?
Ans. Because oxygen atom attracts electrons more strongly than hydrogen, resulting in a partial charge separation.

🔹 Extra Important (41–50)

Q41. Define relative atomic mass.
Ans. The average mass of atoms of an element compared to 1/12th the mass of carbon-12.

Q42. Write one difference between law of constant proportions and law of conservation of mass.
Ans.

• Conservation: Total mass remains constant.
• Constant proportions: Elements combine in fixed ratios by mass.

Q43. Name two compounds containing polyatomic ions.
Ans. Na₂SO₄ (Sodium sulphate), CaCO₃ (Calcium carbonate).

Q44. Write the formula of ammonium sulphate.
Ans. (NH₄)₂SO₄.

Q45. Define gram atomic mass.
Ans. The atomic mass of an element expressed in grams.

Q46. Define gram molecular mass.
Ans. The molecular mass of a substance expressed in grams.

Q47. Write the name of compound with formula KNO₃.
Ans. Potassium nitrate.

Q48. Why do noble gases exist as monoatomic molecules?
Ans. Because they have stable electronic configurations and do not need to combine with other atoms.

Q49. Write any two differences between element and compound.
Ans.

• Element: Made up of one kind of atom.
• Compound: Made up of two or more elements chemically combined.

Q50. Explain why chemical formula of magnesium hydroxide is written as Mg(OH)₂.
Ans. Mg has valency 2, OH has valency 1. To balance, two OH groups combine with one Mg → Mg(OH)₂.