d and f Block Elements — CBSE Class 12 · 2010 · Delhi Board Question with Solution

How would you account for the following?
i) The atomic radii of the metals of the third (5d) series of transition elements are virtually the same as those of the corresponding members of the second (4d) series.
ii) The E° value of the Mn³⁺/Mn²⁺ couple is much more positive than that for the Cr³⁺/Cr²⁺ or Fe³⁺/Fe²⁺ couples.
iii) The highest oxidation state of a metal is exhibited in its oxide or fluoride.

Solution

(i) The 5d series follows the lanthanoids: the lanthanoid contraction exactly offsets the size increase expected on adding a shell, so each 5d radius ≈ the corresponding 4d radius (Zr–Hf, Nb–Ta).
(ii) The reduction Mn

^{3 +}

^{4}

) + e

^{-}

→ Mn

^{2 +}

^{5}

) produces the exactly half-filled, extra-stable configuration, so it is unusually favourable: E° = +1.57 V, far above Cr

^{3 +}

/Cr

^{2 +}

(−0.41 V, which would destroy stable d

^{3}

) and Fe

^{3 +}

/Fe

^{2 +}

(+0.77 V).
(iii) Oxygen and fluorine are the smallest, most electronegative elements; they can pull out the maximum number of metal electrons (oxygen can also form M=O multiple bonds), so the metal reaches its highest state only in oxides and fluorides — e.g. Mn

_{2}

_{7}

(+7), OsO

_{4}

(+8), VF

_{5}

CBSE Class 12 Board · 2010 · Delhi · Subjective

CBSE Class 12 · 2010 · Delhi · d and f Block ElementsShort Answer

Solution

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