CBSE Class 12 Chemistry sample paper 2025-26: question 25

Hints

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Step-by-step solution

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1. I. Chromium and Molybdenum show similar magnetic behaviour.

   ½ mark

2. Cr: [Ar]3d${}^5$4s${}^1$ and Mo: [Kr]4d${}^5$5s${}^1$ have similar configurations, each with 6 unpaired electrons. So both are strongly paramagnetic with similar magnetic moments. (Mg has no unpaired electrons; Fe [Ar]3d${}^6$4s${}^2$ has 4.)

   1 mark

3. II. Zinc and Scandium form white salts.

   ½ mark

4. Zn${}^{2+}$ is [Ar]3d${}^{10}$ (completely filled) and Sc${}^{3+}$ is [Ar]3d${}^0$ (empty), so neither has unpaired d electrons. No d–d transition is possible, so their salts are white. Ni${}^{2+}$ (3d${}^8$) and V ions (e.g. V${}^{3+}$, 3d${}^2$) have unpaired electrons, so their salts are coloured.

   1 mark

Answer: I. Cr and Mo (similar ns¹(n−1)d⁵ configuration, 6 unpaired electrons each). II. Zn and Sc (Zn²⁺ 3d¹⁰, Sc³⁺ 3d⁰: no unpaired electrons, no d–d transition).

Revise the chapter

• Chapter: The d- and f-Block Elements (notes, formulas, where marks are lost and every past question)
• Topic: Magnetic Properties and Colour of Transition Metal Ions