MP Board Solutions Class 9 Science Chapter 3 – Atoms and Molecules

Atoms and Molecules – Important Questions with Answers

1. Define atom and molecule with one example each.
Answer: An atom is the smallest unit of an element (e.g., O). A molecule is two or more atoms chemically bonded (e.g., H₂O).

2. State any four postulates of Dalton’s atomic theory.
Answer: All matter consists of indivisible atoms; atoms of the same element are identical; compounds form from fixed atom ratios; atoms are rearranged, not created/destroyed in reactions.

3. What is atomicity? Give two examples.
Answer: Number of atoms in a molecule (e.g., N₂ = 2, P₄ = 4).

4. Define valency with examples: O, H, C.
Answer: The combining capacity of an atom (O = 2, H = 1, C = 4).

5. Calculate molecular mass of CO₂.
Answer: 12 + (16 × 2) = 44 u.

6. What is the difference between molar mass and molecular mass?
Answer: Molecular mass is measured in atomic mass units (u); molar mass is grams per mole.

7. Define mole and Avogadro’s number.
Answer: One mole contains 6.022×10²³ particles (Avogadro’s constant).

8. How many molecules are present in 3 moles of O₂?
Answer: 3 × 6.022×10²³ = 1.8066×10²⁴ molecules.

9. Number of atoms in 1 mole of CH₄.
Answer: 5 atoms per molecule → 5 × 6.022×10²³ = 3.011×10²⁴ atoms.

10. Define isotopes with one example.
Answer: Atoms of the same element with different masses, e.g., ¹H & ²H.

11. What are isobars? Give one example.
Answer: Atoms with same mass number but different atomic numbers, e.g., ⁴⁰Ar and ⁴⁰Ca.

12. Define isomers with one example.
Answer: Compounds with same molecular formula but different structures, e.g., ethanol and dimethyl ether.

13. What is the atomic number?
Answer: Number of protons in the nucleus of an atom.

14. What is mass number?
Answer: Total number of protons and neutrons in an atom’s nucleus.

15. Define electron configuration.
Answer: Distribution of electrons in various shells of an atom.

16. Electronic configuration of Neon (Z = 10).
Answer: 2, 8.

17. Electronic configuration of Carbon (Z = 6).
Answer: 2, 4.

18. What is the significance of valence electrons?
Answer: They determine an element’s chemical bonding behavior.

19. What holds atoms together in a molecule?
Answer: Chemical bonds (e.g., ionic, covalent bonds).

20. Define formula unit mass.
Answer: Sum of atomic masses in the simplest ratio of ions in an ionic compound.

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

22. State law of definite proportions.
Answer: A compound always contains elements in a fixed ratio by mass.

23. How many oxygen atoms are there in 2 moles of CO₂?
Answer: Each molecule has 2 O atoms → 2 × 2 × 6.022×10²³ = 2.4088×10²⁴ atoms.

24. What is the empirical formula of C₂H₆?
Answer: CH₃ (simplest whole-number ratio).

25. Distinguish between molecule and formula unit.
Answer: Molecule applies to covalent compounds; formula unit applies to ionic compounds.