How would you account for the following?
i) Many of the transition elements and their compounds can act as good catalysts.
ii) The metallic radii of the third (5d) series of transition elements are virtually the same as those of the corresponding members of the second series.
iii) There is a greater range of oxidation states among the actinoids than among the lanthanoids.
i) Many of the transition elements and their compounds can act as good catalysts.
ii) The metallic radii of the third (5d) series of transition elements are virtually the same as those of the corresponding members of the second series.
iii) There is a greater range of oxidation states among the actinoids than among the lanthanoids.
Solution
(i) Transition metals cycle between oxidation states, forming transient intermediates that provide lower-activation-energy routes; their surfaces (vacant d-orbitals) also adsorb and activate reactants — hence catalysts like Fe, Ni, Pt, VO.
(ii) The lanthanoid contraction — poor shielding by the 14 intervening 4f electrons — cancels the expected size increase from the 4d to the 5d series, leaving the radii of corresponding members virtually identical.
(iii) Actinoid 5f, 6d and 7s orbitals lie extremely close in energy; varying numbers of electrons can be lost or shared, giving states from +3 to +7, whereas buried lanthanoid 4f electrons confine them essentially to +3.
(ii) The lanthanoid contraction — poor shielding by the 14 intervening 4f electrons — cancels the expected size increase from the 4d to the 5d series, leaving the radii of corresponding members virtually identical.
(iii) Actinoid 5f, 6d and 7s orbitals lie extremely close in energy; varying numbers of electrons can be lost or shared, giving states from +3 to +7, whereas buried lanthanoid 4f electrons confine them essentially to +3.
CBSE Class 12 Board · 2009 · Subjective