NCERT Solutions Class 9 Science Chapter 3 - Atoms and Molecules

Atoms and Molecules – Important Questions with Answers

1. Define atom.
Answer: An atom is the smallest particle of an element that retains the chemical properties of that element.

2. Define molecule. Give two examples.
Answer: A molecule is a group of two or more atoms chemically bonded together.
Examples: Oxygen (O₂), Water (H₂O).

3. What is a compound? Give two examples.
Answer: A compound is a substance formed when two or more elements chemically combine in a fixed ratio.
Examples: Water (H₂O), Carbon dioxide (CO₂).

4. State the postulates of Dalton’s atomic theory.
Answer:

• All matter is made up of indivisible particles called atoms.

• Atoms of the same element are identical in mass and properties.

• Atoms of different elements have different masses and properties.

• Atoms combine in simple whole-number ratios to form compounds.

• Atoms cannot be created or destroyed in chemical reactions.

5. What is meant by atomicity? Give examples.
Answer: Atomicity is the number of atoms present in a molecule.
Examples: Oxygen (O₂) – Diatomic, Phosphorus (P₄) – Tetratomic

6. What is a chemical formula?
Answer: A chemical formula is a symbolic representation of a compound using the symbols of its elements and the ratio in which they combine.

7. Write the chemical formula for the following:
a) Ammonia
b) Carbon dioxide
c) Calcium chloride
Answer:
a) NH₃
b) CO₂
c) CaCl₂

8. What do you understand by valency?
Answer: Valency is the combining capacity of an element.

9. Find the valency of the following elements: Hydrogen, Oxygen, Nitrogen, Carbon.
Answer:
Hydrogen – 1
Oxygen – 2
Nitrogen – 3
Carbon – 4

10. Define ion. Give examples.
Answer: An ion is a charged particle formed by the gain or loss of electrons.
Examples: Na⁺ (positive ion), Cl⁻ (negative ion)

11. Differentiate between cation and anion.
Answer:

• Cation is a positively charged ion (e.g., Na⁺).

• Anion is a negatively charged ion (e.g., Cl⁻).

12. What is a molecular mass?
Answer: Molecular mass is the sum of the atomic masses of all the atoms present in a molecule.

13. Calculate the molecular mass of CO₂.
Answer:
C = 12 u, O = 16 u
Molecular mass of CO₂ = 12 + (16 × 2) = 44 u

14. What is a mole?
Answer: A mole is the quantity of substance containing 6.022 × 10²³ particles (Avogadro’s number).

15. What is Avogadro’s constant?
Answer: Avogadro’s constant is 6.022 × 10²³, the number of particles in one mole of a substance.

16. How many molecules are there in 2 moles of oxygen gas?
Answer: 1 mole = 6.022 × 10²³ molecules
So, 2 moles = 2 × 6.022 × 10²³ = 1.2044 × 10²⁴ molecules

17. What is the mass of 1 mole of water (H₂O)?
Answer: H = 1 u, O = 16 u
Molar mass = 2 × 1 + 16 = 18 g

18. Write the chemical names of the following compounds:
a) HCl
b) NaOH
c) CaCO₃
Answer:
a) Hydrogen chloride
b) Sodium hydroxide
c) Calcium carbonate

19. What is the formula unit mass?
Answer: The formula unit mass is the sum of atomic masses of the ions in an ionic compound.

20. What is the difference between molecular mass and molar mass?
Answer:

• Molecular mass is measured in atomic mass units (u).

• Molar mass is the mass of 1 mole of a substance in grams.

21. State the law of conservation of mass.
Answer: Mass can neither be created nor destroyed in a chemical reaction.

22. State the law of constant proportions.
Answer: A chemical compound always contains the same elements in the same fixed ratio by mass.

23. Calculate the number of atoms in 1 mole of methane (CH₄).
Answer: CH₄ has 5 atoms (1 C + 4 H)
1 mole of CH₄ = 6.022 × 10²³ molecules
Total atoms = 5 × 6.022 × 10²³ = 3.011 × 10²⁴ atoms

24. Write the chemical formula of:
a) Aluminium oxide
b) Magnesium chloride
c) Potassium nitrate
Answer:
a) Al₂O₃
b) MgCl₂
c) KNO₃

25. Name the following compounds:
a) Na₂SO₄
b) KCl
c) Ca(OH)₂
Answer:
a) Sodium sulphate
b) Potassium chloride
c) Calcium hydroxide