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Assertion (A) : Actinoids… (Class 12 Chemistry 2025, Q15)

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ChemistryCBSE Class 12 · Board exam 2025, set 56/7/1 · 1 mark · Assertion-reason

CBSE Class 12 Chemistry 2025: question 15

Set in the CBSE Class 12 board exam of 2025, paper 56/7/1, section A, question 15, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q15 · 1 mark · Assertion-reasonCompetency-basedThe d- and f-Block Elements

Assertion (A) : Actinoids show irregularities in their electronic configurations.
Reason (R) : Actinoids are radioactive in nature.

  1. (A)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
  2. (B)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
  3. (C)Assertion (A) is true, but Reason (R) is false.
  4. (D)Assertion (A) is false, but Reason (R) is true.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 56/7/1.

Hints

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

Hint 1

What causes irregular electronic configurations?

Hint 2

Is radioactivity related to orbital energies?

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. Actinoids show irregular configurations because the 5f, 6d and 7s orbitals have comparable energies.

  2. Actinoids are also radioactive, but radioactivity does not explain their configurations. Both statements are true and R is not the correct explanation.

    1 mark

Answer: (B)

Where the marks are

  • Both true, but R does not explain A (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from The d- and f-Block Elements, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 56/7/1: 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.