(a) Account for the following : 

(i) Copper (I) compounds are white whereas Copper (II) compounds are coloured.

(ii) Chromates change their colour when kept in an acidic solution.

(iii) Zn, Cd, Hg are considered as d-block elements but not as transition elements.

(b) Calculate the spin-only moment of Co^{2+} (Z = 27) by writing the electronic configuration of Co and Co^{2+}.

 

 
 
 
 
 

Answers (1)

(a)

(i) Copper (I) compounds are white whereas Copper (II) compounds are coloured because, in Cu+1(3d10) there is absence of unpaired electrons while in Cu+2 (3d9 ) compounds are coloured due to unpaired e- shows d-d transition.

(ii) chromate (CrO42- ) changes to dichromate (Cr2O72- ) ion in acidic medium, hence color is changed. 

(iii) Due to completely filled d-orbitals in their ground state as well as in oxidized state,  Zn, Cd, Hg are considered as d-block elements but not as transition elements.

(b) Co = [Ar]4s2 3d7 , Co^{2+} =[Ar] 3d7

\mu = \sqrt{n(n+2)} = \sqrt{3(3+2)} = \sqrt{15}

= 3.2 BM 

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