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(i) Draw the number of… (Class 12 Physics 2026, Q26b)

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PhysicsCBSE Class 12 · Board exam 2026, set 55/5/1 · 3 marks · Short answer

CBSE Class 12 Physics 2026: question 26(b)

Set in the CBSE Class 12 board exam of 2026, paper 55/5/1, section C, question 26(b), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26(b) · 3 marks · Short answerAtoms

(i) Draw the number of scattered particles versus the scattering angle graph for scattering of alpha particles by a thin foil. Write two important conclusions that can be drawn from this plot.
(ii) If Bohr's quantization postulate (angular momentum = ) is a basic law of nature, it should be equally valid for the case of planetary motion also. Why, then, do we never speak of quantization of orbits of planets around the Sun ? Explain.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 55/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Most α-particles go nearly straight through — what happens at large angles?

Hint 2

Estimate n = 2πmvr/h for the Earth's orbit and consider how close successive levels would be.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (i) Graph: number of scattered α-particles N(θ) (log scale) on y-axis vs scattering angle θ (0° to 180°) on x-axis — very large at small angles and falling steeply as θ increases (), with only a very few particles at angles close to 180°.

    1 mark

  2. Conclusions:
    • Most α-particles pass undeflected, so most of the atom is empty space.
    • A very few are deflected through large angles (some > 90°), so the positive charge and nearly all the mass of the atom are concentrated in a tiny central nucleus.

    1 mark

  3. (ii) Angular momentum of a planet is enormous: for the Earth, J s, so . For such huge n, the difference between successive allowed angular momenta and energies is negligibly small compared to the values themselves, so the allowed orbits appear continuous; quantization is unobservable and is not spoken of.

    1 mark

Answer: (i) N(θ) falls sharply with θ; atom mostly empty, positive charge and mass concentrated in tiny nucleus; (ii) n ~ 10^74 for planets, so quantized levels are practically continuous

Where the marks are

  • Correct graph of N vs θ (1 mark)
  • Two conclusions: atom mostly empty; tiny massive positive nucleus (½ + ½ mark)
  • Explains huge quantum number (~10⁷⁴) makes quantization unnoticeable (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Atoms, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 55/5/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.