Account for the following:
(i) Copper(I) compounds are white whereas Copper (II) compounds are colored.
(ii) Chromates change their color when kept in an acidic solution.
(iii) Zn, Cd, Hg are considered as d-block elements but not as transition elements.

Solution:

(i) Copper (I) compounds are white whereas Copper (II) compounds are colored because, in ${\mathrm{Cu}}^{+1}\left(3{\mathrm{d}}^{10}\right)$ there is absence of unpaired electrons while in ${\mathrm{Cu}}^{+2}\left(3{\mathrm{d}}^9\right)$ compounds are colored due to unpaired e- shows d-d transition.
(ii) Chromate $\left({{\mathrm{CrO}}_4}^{2-}\right)$ changes to dichromate $\left({{\mathrm{Cr}}_2{\mathrm{O}}_7}^{2-}\right)$ 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.