d and f Block Elements — CBSE Class 12 · 2017 Board Question with Solution

(a) Account for the following:
i) Transition metals show variable oxidation states.
ii) Zn, Cd and Hg are soft metals.
iii) The E° value for the Mn³⁺/Mn²⁺ couple is highly positive (+1.57 V) as compared to Cr³⁺/Cr²⁺.
(b) Write one similarity and one difference between the chemistry of lanthanoid and actinoid elements.

Solution

(a)(i) The (n−1)d and ns orbitals are so close in energy that different numbers of electrons can be used in bonding — the states vary in unit steps.
(ii) Zn, Cd and Hg have the d

^{10}

^{2}

configuration: with no unpaired d electrons, the metallic bonding is weak — they are soft and low-melting.
(iii) Mn

^{3 +}

+ e

^{-}

→ Mn

^{2 +}

creates the half-filled 3d

^{5}

(extra stable), so the reduction is highly favoured (+1.57 V). Cr

^{3 +}

+ e

^{-}

→ Cr

^{2 +}

would destroy the stable t

_{2 g}^{3}

of Cr

^{3 +}

, so that couple is negative (−0.41 V).
(b) Similarity: both series predominantly show the +3 oxidation state and exhibit a steady contraction in radii.
Difference: actinoids display a much wider range of oxidation states (+3 to +7) owing to near-degenerate 5f/6d/7s orbitals, and all are radioactive — lanthanoids are essentially +3 and (except Pm) non-radioactive.

CBSE Class 12 Board · 2017 · Subjective

CBSE Class 12 · 2017 · d and f Block ElementsLong Answer

Solution

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