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I. Why hydrogen… (Class 12 Physics SQP 2026-27, Q21A)

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PhysicsCBSE Class 12 · Sample paper, session 2026-27 · 2 marks · Very short answer

CBSE Class 12 Physics sample paper 2026-27: question 21(A)

From the official CBSE sample question paper for 2026-27, section B, question 21(A), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21(A) · 2 marks · Very short answerAtoms

I. Why hydrogen spectral lines are considered to be a fingerprint of the element?
II. If energy of an electron of the hydrogen atom in ground state is -13.6 eV, what should be the minimum energy of photon which can ionize the atom?

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

Spectral lines come from transitions between energy levels — are these levels the same for every element?

Hint 2

Ionisation means taking the electron to the level where energy is zero.

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. Hydrogen has its own unique set of quantised energy levels; each line corresponds to a definite transition . No other element has the same set of wavelengths, so the pattern of lines identifies hydrogen uniquely, like a fingerprint.

    1 mark

  2. II. To ionise, the electron must be raised from eV to . Minimum photon energy eV.

    1 mark

Answer: I. Unique characteristic line spectrum; II. 13.6 eV

Where the marks are

  • Unique discrete energy levels give a characteristic line spectrum (1 mark)
  • Minimum ionising photon energy = 13.6 eV (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 paperSample paper 2026-27: 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.