d and f Block Elements — CBSE Class 12 · 2007 · Delhi; also All India 2005 Board Question with Solution

a) Describe the general trends in the following properties of the first series of transition elements:
i) stability of the +2 oxidation state
ii) formation of oxometal ions
b) Assign a reason for each of the following:
i) Transition elements exhibit variable oxidation states.
ii) Transition metal ions are usually coloured.

Solution

(a)(i) Stability of +2: across Sc → Zn the +2 state becomes progressively more stable as the growing nuclear charge makes removal of further d electrons harder; superimposed are humps of extra stability at Mn

^{2 +}

(3d

^{5}

) and Zn

^{2 +}

(3d

^{10}

), while early members (Sc, Ti, V) prefer higher states.
(ii) Oxometal ions: in their high oxidation states the early/middle metals bond to oxygen through strong pπ–dπ multiple bonds, giving oxocations and oxoanions such as VO

^{2 +}

, VO

_{2}^{+}

, CrO

_{4}^{2 -}

/Cr

_{2}

_{7}^{2 -}

and MnO

_{4}^{-}

; the tendency peaks where the oxidation state is highest (Cr +6, Mn +7).
(b)(i) The (n−1)d and ns orbitals are nearly equal in energy, so a variable number of electrons participates in bonding — states change in unit steps.
(ii) Their ions have partly filled d-orbitals; ligand-field splitting allows d–d transitions that absorb visible light, and the transmitted complement appears as the colour.

CBSE Class 12 Board · 2007 · Delhi; also All India 2005 · Subjective

CBSE Class 12 · 2007 · Delhi; also All India 2005 · d and f Block ElementsLong Answer

Solution

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