d and f Block Elements — CBSE Class 12 · 2022 Board Question with Solution

(a) Account for the following:
i) Copper(I) compounds are white whereas copper(II) compounds are coloured.
ii) Chromates change their colour when kept in an acidic solution.
iii) Zn, Cd and Hg are considered d-block elements but not transition elements.
(b) Calculate the spin-only magnetic moment of Co²⁺ (Z = 27) by writing the electronic configurations of Co and Co²⁺.

Solution

(a)(i) Cu

^{+}

has the filled 3d

^{10}

shell — no d–d transition, hence white; Cu

^{2 +}

(3d

^{9}

) absorbs visible light through d–d transitions and is coloured.
(ii) In acid, yellow chromate converts to orange dichromate:

2Cr O_{4}^{2 -} + 2 H^{+} \to C r_{2} O_{7}^{2 -} + H_{2} O

(iii) Zn, Cd, Hg belong to the d-block by position, but their atoms and M

^{2 +}

ions all have completely filled d

^{10}

subshells — they never have a partly filled d-orbital, so they are not transition elements.
(b) Co: [Ar]3d

^{7}

^{2}

→ Co

^{2 +}

: [Ar]3d

^{7}

.
In 3d

^{7}

there are 3 unpaired electrons:

μ = n (n + 2) = 3 \times 5 = 15 \approx 3.87 BM

CBSE Class 12 Board · 2022 · Subjective

CBSE Class 12 · 2022 · d and f Block ElementsLong Answer

Solution

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