Chemistry · d- and f-block elements · chapter 4 of 10

Class 12 Chemistry Chapter 4: The d- and f-Block Elements MCQs with answers

10 multiple-choice questions on The d- and f-Block Elements, each with its answer and the reason, written from the NCERT chapter. For the CBSE board, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.

Question 1
The outer electronic configuration of copper (Z = 29) in its ground state is
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Answer: (d) 3d¹⁰4s¹

A completely filled 3d set is relatively more stable, so one 4s electron moves into 3d: Cu is [Ar]3d¹⁰4s¹, not 3d⁹4s².

Question 2
The electronic configuration of the Co³⁺ ion (Z of cobalt = 27) is
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Answer: (a) [Ar]3d⁶

Co is [Ar]3d⁷4s². On ionisation the two 4s electrons leave first, then one 3d electron: Co³⁺ is [Ar]3d⁶, with no 4s electrons.

Question 3
Which of these ions has an incomplete d subshell, so that its metal counts as a transition element?
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Answer: (a) Ag²⁺

A transition element has an incompletely filled d subshell in at least one of its common oxidation states. Zn²⁺, Cd²⁺ and Hg²⁺ are all d¹⁰; Ag²⁺ is 4d⁹, so silver, through Ag(II), qualifies while zinc, cadmium and mercury do not.

Question 4
The highest oxidation state shown by vanadium (Z = 23) in its compounds is
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Answer: (d) +5

Up to manganese the maximum state equals the number of s plus d electrons: V is 3d³4s², 3 + 2 = 5, as in VO₂⁺.

Question 5
Ferrate ion, FeO₄²⁻, is formed in alkaline medium. The oxidation state of iron in it is
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Answer: (a) +6

x + 4(−2) = −2, so x = +6. Ferrate(VI) decomposes readily to Fe₂O₃ and O₂, which is why no iron oxide above Fe₂O₃ is known.

Question 6
An aqueous solution of a salt of which of the following ions is pink?
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Answer: (c) Mn²⁺

Aquated Mn²⁺ is pink; Ni²⁺ is green, Cr³⁺ violet and Cu²⁺ blue.

Question 7
Which set contains only ions that are colourless in aqueous solution?
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Answer: (d) Sc³⁺, Ti⁴⁺, Zn²⁺

Sc³⁺ and Ti⁴⁺ are 3d⁰ and Zn²⁺ is 3d¹⁰: none can have a d–d transition. Ti³⁺ (3d¹, purple), Fe³⁺ (3d⁵, yellow) and Cu²⁺ (3d⁹, blue) are coloured.

Question 8
Which pair of aquated ions has the same spin-only magnetic moment?
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Answer: (d) Ti²⁺ and Ni²⁺

μ = √(n(n+2)) BM for n unpaired electrons. Ti²⁺ (d²) and Ni²⁺ (d⁸) both have two unpaired electrons: √8 = 2.84 BM. V²⁺ (d³) 3.87 against Mn²⁺ (d⁵) 5.92; Cr²⁺ (d⁴) 4.90 against Fe³⁺ (d⁵) 5.92; Co²⁺ (d⁷) 3.87 against Cu²⁺ (d⁹) 1.73.

Question 9
A tripositive ion M³⁺ of a 3d metal has a spin-only magnetic moment of 3.87 BM. The metal M is
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Answer: (d) chromium (Z = 24)

3.87 BM = √(3 × 5), so n = 3. M³⁺ has (Z − 21) d electrons: V³⁺ d² (2 unpaired, 2.84), Cr³⁺ d³ (3 unpaired, 3.87), Mn³⁺ d⁴ (4, 4.90), Fe³⁺ d⁵ (5, 5.92). Only chromium fits.

Question 10
Which pair of elements has almost identical atomic radii?
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Answer: (a) Zr and Hf

Because of the lanthanoid contraction the 4d and 5d members of a group have nearly the same radii: Zr 160 pm, Hf 159 pm. Down from 3d to 4d (Ti to Zr, Sc to Y, Cu to Ag) the radius increases.

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