d and f Block Elements — CBSE Class 12 · 2006 · All India Compartment Board Question with Solution

a) Explain why transition elements show variable oxidation states. Write all the possible oxidation states of an element having atomic number 25.
b) Compare the chemistry of the actinoids with that of the lanthanoids in reference to:
i) atomic and ionic sizes
ii) oxidation states

Solution

(a) In transition elements the (n−1)d and ns orbitals are very close in energy; besides the ns electrons, different numbers of d electrons can also take part in bonding — the oxidation state therefore varies in unit steps.
Z = 25 is manganese, [Ar]3d

^{5}

^{2}

. Its oxidation states:

+ 2, + 3, + 4, + 5, + 6, + 7

(e.g. MnCl

_{2}

, Mn

_{2}

_{3}

, MnO

_{2}

, MnO

_{4}^{3 -}

, MnO

_{4}^{2 -}

, MnO

_{4}^{-}

).
(b)(i) Atomic/ionic sizes: both series show a steady contraction with atomic number (lanthanoid and actinoid contraction); the actinoid contraction is the greater, element for element, because 5f electrons shield even more poorly than 4f.
(ii) Oxidation states: the lanthanoids are essentially limited to +3 (occasional +2, +4 near f

^{0}

^{7}

^{14}

). The actinoids, whose 5f, 6d and 7s orbitals are nearly degenerate, show a far wider range — +3 up to +7 (U +6, Np +7).

CBSE Class 12 Board · 2006 · All India Compartment · Subjective

CBSE Class 12 · 2006 · All India Compartment · d and f Block ElementsLong Answer

Solution

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