(a) 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.

(b) Calculate the spin-only moment of ${\mathrm{Co}}^{2+}(\mathrm{Z}=27)$ by writing the electronic configuration of Co and Co²⁺.

OR

(a) Give three points of difference between lanthanoids and actinoids

(b) Give reason and select one atom/ion which will exhibit asked property:
${\mathrm{Sc}}^{3+} \mathrm{or} {\mathrm{Cr}}^{3+}$ (Exhibit diamagnetic behavior)
Cr or Cu (High melting and boiling point)

(a) (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.

(b) $\mathrm{Co}=\left[\mathrm{Ar}\right]4{\mathrm{s}}^{2}3{\mathrm{d}}^7,{\mathrm{Co}}^{2+}=\left[\mathrm{Ar}\right]3{\mathrm{d}}^7$
$\begin{array}{l}\mathrm{\mu }=\sqrt{\mathrm{n}(\mathrm{n}+2)}=\sqrt{3(3+2)}=\sqrt{15}\\ =3.2\mathrm{BM}\end{array}$

OR

(a) Lanthanoids
(i)Most of them are not radioactive
(ii)Don’t show a wide range of oxidation state
(iii)Most of their ions are colorless
Actinoids
(i)All are radioactive
(ii)Show a wide range of oxidation states
(iii)Most of their ions are colored

(b) (i) Sc⁺³ exhibit diamagnetic behavior, because of absence of unpaired electron.
(ii) Cr has high melting and boiling point, because of presence of strong intermetallic bonding.