Question

Out of Fe$^{3+}$, Sc$^{3+}$, Cr$^{3+}$ and Co$^{3+}$ ions, the one which is colourless in aqueous solution is:

• A. Sc$^{3+}$
• B. Fe$^{3+}$
• C. Cr$^{3+}$
• D. Co$^{3+}$

Hint:

For transition metal ions, remember that the presence of unpaired electrons in the d-orbital often results in colored solutions.

Answer 1

The Correct Option is A

Sc$^{3+}$ ion has no unpaired electrons in its d-orbitals, and thus does not absorb visible light, making it colorless in aqueous solution. The other ions have d-orbitals with unpaired electrons, resulting in colored solutions.