Question

What is the formula of brown ppt?
(A) $C{u_2}{I_2}$
(B) $C{u_2}{I_2} + {I_3}^ -$
(C) $Cu{I_2}$
(D) $CuS{O_4}$

Answer 1

Copper is a transition metal with the ability to show variable oxidation states and also d-d transitions that are responsible for imparting colour to its compounds and precipitates. But all oxidation states of copper cannot show d-d transitions.

Complete answer:
The electronic configuration of copper in its $+ 1$ oxidation state is $3{d^{10}}$ and that of copper in its $+ 2$ oxidation state is $3{d^9}$ . Thus, copper in its $+ 2$ oxidation state has one vacant space in the d orbital for an electron but copper in its $+ 1$ oxidation state has no vacant spaces and a completely filled d-orbital.
Transition metals ions that have a fully filled d-orbital are incapable of showing d-d transitions that are mainly responsible for imparting colour to these ions. Each ion in its particular oxidation state shows a characteristic colour depending upon the wavelength of the d-d transitions that take place.
Salts formed by copper $C{u^{2 + }}$ ions have a characteristic blue colour due to the presence of d-d transitions associated with the partially filled d-orbital. Hence, $Cu{I_2}$ and $CuS{O_4}$ are blue coloured salts.
The copper ions in $+ 1$ oxidation state are mainly colorless or white due to the absence of d-d transitions. Therefore a copper salt alone cannot form a brown precipitate. But iodine gas in the presence of potassium iodide forms triiodide ions which are initially yellow but turn dark brown in colour upon standing.
${I_2} + KI \to {I_3}^ -$
These triiodide ions get adsorbed on the surface of colourless precipitate of copper resulting in a coloured precipitate
$(C{u_2}{I_2} + {I_3}^ - )$ this combination is brown in colour.
Hence, the correct option is (b) and the brown precipitate is due to the formation of $(C{u_2}{I_2} + {I_3}^ - )$ .

Note:
Not all transition metal compounds and complexes are coloured solely due to the d-d transitions. Many complexes get their colour due to the presence of ligands that are conjugated in nature or due to charge transfer phenomenon.