Atoms: Class 12 Important Questions with Answers

Practice set: test yourself

Try every question before looking at the answers below. 64 or more out of 80 means you are ready for the board.

C. Very short answer (1 mark each)

1. Define impact parameter.
2. Define ionisation energy.
3. Energy of H in n = 3?
4. Who explained Bohr's second postulate?
5. Name the series ending on n = 3.
6. What does the distance of closest approach estimate?
7. Value of PE in the ground state of H?
8. How does the radius depend on n?
9. Why is the energy negative?
10. Give one limitation of Bohr's model.

D. Short answer I (2 marks each)

1. Write two conclusions of the α-scattering experiment.
2. Derive d for an α-particle of KE K.
3. Find the wavelength of the 4 → 2 transition.
4. Show T ∝ n³.
5. Explain how de Broglie's idea gives L = nh/2π.
6. Find the energy needed to take an electron from n = 2 to n = 5.
7. Why does Rutherford's model fail? Give two reasons.
8. Find the speed and radius in n = 3.

Rough work

E. Short answer II (3 marks each)

1. State Bohr's postulates and derive the radius of the nth orbit.
2. Derive the total energy of the electron in the nth orbit; explain the negative sign.
3. Draw the energy level diagram of H and mark the Lyman, Balmer and Paschen series.
4. An electron in H falls from n = 3 to n = 2. Find (a) the photon energy, (b) λ, (c) the change in the radius.
5. Describe the Geiger–Marsden experiment with a diagram.
6. Show that the energy levels crowd together at large n.

F. Long answer (5 marks each)

1. Describe Rutherford's α-scattering experiment. Draw the N–θ graph and the trajectories for different impact parameters. What conclusions follow? Find the closest approach of a 7.7 MeV α-particle to gold.
2. State Bohr's postulates. Derive expressions for rₙ, vₙ and Eₙ. Find the wavelength of the first line of the Balmer series. Give two limitations of the model.

G. HOTS (higher-order thinking)

1. Why is gold used in the α-scattering experiment?
2. Why can't an electron in H have an energy of −5 eV?
3. An atom absorbs 10.2 eV. Which level does it reach and how many lines can it then emit?
4. If the electron in H were replaced by a particle 200 times heavier with the same charge, how would a₀ change?

20 MCQs

1. Bohr radius
   (a) 5.3 Å (b) 1.06 Å (c) 0.53 Å (d) 0.053 Å
2. Energy of H in n = 2
   (a) −3.4 eV (b) −13.6 eV (c) −1.51 eV (d) −6.8 eV
3. rₙ ∝
   (a) n (b) n³ (c) 1/n (d) n²
4. vₙ ∝
   (a) n (b) 1/n (c) n² (d) 1/n²
5. T ∝
   (a) n (b) n³ (c) n² (d) 1/n³
6. Lyman series region
   (a) IR (b) visible (c) microwave (d) UV
7. Head-on collision: θ =
   (a) 180° (b) 90° (c) 0° (d) 45°
8. Ionisation energy of H
   (a) 10.2 eV (b) 3.4 eV (c) 13.6 eV (d) 27.2 eV
9. In the ground state, PE
   (a) 13.6 eV (b) 27.2 eV (c) −13.6 eV (d) −27.2 eV
10. Angular momentum in n = 3
    (a) h/2π (b) 3h/2π (c) 3h (d) h/3π
11. Most of the atom is
    (a) nucleus (b) electrons (c) empty space (d) protons
12. Second Balmer line
    (a) 4 → 2 (b) 3 → 2 (c) 4 → 3 (d) 2 → 1
13. d ∝ (for fixed Z)
    (a) K (b) 1/K (c) K² (d) √K
14. Gold foil experiment by
    (a) Thomson (b) Bohr (c) Geiger and Marsden (d) de Broglie
15. Number of lines from n = 4 to 1
    (a) 3 (b) 4 (c) 10 (d) 6
16. Energy to go from n = 1 to 2
    (a) 10.2 eV (b) 13.6 eV (c) 3.4 eV (d) 12.09 eV
17. 2πrₙ equals
    (a) nλ (b) λ/n (c) n²λ (d) λ
18. K = ? in terms of E
    (a) E (b) −2E (c) 2E (d) −E
19. Paschen series ends on
    (a) n = 1 (b) n = 3 (c) n = 2 (d) n = 4
20. Thomson's model is called
    (a) planetary (b) nuclear (c) plum pudding (d) wave

1 (c) · 2 (a) · 3 (d) · 4 (b) · 5 (b) · 6 (d) · 7 (a) · 8 (c) · 9 (d) · 10 (b) · 11 (c) · 12 (a) · 13 (b) · 14 (c) · 15 (d) · 16 (a) · 17 (a) · 18 (d) · 19 (b) · 20 (c)

Assertion–Reason (6)

(a) Both true, R explains A. (b) Both true, R does not explain A. (c) A true, R false. (d) A false, R true.

1. A: Very few α-particles bounce back from gold foil. R: The nucleus is very small compared with the atom.
2. A: The total energy of the electron in H is negative. R: The electron is bound to the nucleus.
3. A: The speed of the electron is greatest in n = 1. R: vₙ ∝ 1/n.
4. A: Bohr's model explains the spectrum of helium. R: Bohr's model ignores electron–electron repulsion.
5. A: Energy levels of H are equally spaced. R: Eₙ ∝ 1/n².
6. A: Balmer lines are visible. R: Balmer transitions end on n = 2.

Key: 1 (a) · 2 (a) · 3 (a) · 4 (d) · 5 (d) · 6 (a)

Answers to the practice set