1.

Explain the following observations:(i) The enthalpies of atomisation of transition metals are quite high.(ii) There is a close similarity in physical and chemical properties of the 4 d and 5 d series of the transition elements, much more than expected on the basis of usual family relationship.(iii) The members in the actinoid series exhibit larger number of oxidation states than the corresponding members in the lanthanoid series.

Answer»

(i) This is because transition metals have strong metallic bonds as they have large number of unpaired electrons.
(ii) There is a close similarity in physical and chemical properties of the 4d and 5d series of the transition elements much more than expected on the basis of usual family relationship. This is because 5d and 4d series elements have virtually the same atomic and ionic radii due to lanthanide contraction. Due to equality in size of Zr and Hf, Nb and Ta, Mo and W, etc., the two elements of pair have the same properties.
(iii) The members in the actinoid series exhibit larger number of oxidation states than the corresponding members in the lanthanide series due to the fact that the 5f, 6d and 7s levels are of comparable energies.


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