Question

Which of the following anions is orange in color?
(A) $HSO_4^-$​
(B) $MnO_4$​
(c) $Cr_2O_7^{2-}$​​
(D) All of the above

Answer 1

Color in an ion is possible only if it contains a transition metal in it. Transition metals are the $d-$block elements that lie between the metals and nonmetals in the periodic table. They show a transition from metals to nonmetals. Hence, the name transition elements.

Complete step by step answer:
Among the given options, $Cr_2O_7^{2-}$ is orange in color. $Cr_2O_7^{2-}$ anion is called the dichromate ion. Transition metal ions show color due to the presence of incompletely or half-filled $d-$orbitals. Let us check the filling of $d-$ block orbitals in the chromium ion. The atomic number of chromium is $24$. Its electronic configuration is $[Ar]3d^6$. So, it has an incompletely filled $d-$orbital. When a metal forms a bond with the anion, the degenerate $d-$ orbitals which were earlier in the same energy level, now get split into two different energy levels, namely, $t_2g$ and $e_g$. The electrons now jump from the lower $t_2g$ orbital to the higher $e_g$ orbital. In doing so, the electron absorbs energy. Now, an electron cannot stay in the higher orbital and tends to jump back to its lower orbital immediately by releasing the absorbed energy. In the case of dichromate ions, the color released by the chromium ion while jumping from a higher $t_2g$ orbital to a lower $e_g$ orbital is orange. Therefore, the anion that is orange in color is $Cr_2O_7^{2-}$.

So, the correct answer is Option C .

Note: 1.The $d-$orbital has $5$ degenerate orbitals. Degenerate orbitals are the orbitals that have the same energy. When a metal approaches a transition metal ion, the five degenerate $d-$orbitals split into three higher and two lower energy level orbitals, called the generated orbitals. This is the principle of the crystal field theory.
2.The anion $HSO_4^-$ consists of the metal ion, $H^+$, which is not a transition metal. Hence, the anion does not show any color.
3.The anion $MnO_4^-$ consists of $Mn^{+7}$ as the transition metal ion, but it is not orange in color. The anion is purple.