Question

Which cation from the following does NOT form a colourless compound? (Atomic number: Cu = 29, Ti = 22, Zn = 30, Sc = 21)

• A. $Sc^{3+}$
• B. $Cu^{+}$
• C. $Ti^{3+}$
• D. $Zn^{2+}$

Hint:

Transition metal ions with $d^0$ or $d^{10}$ configurations usually form colourless compounds, while partially filled $d$ orbitals lead to coloured compounds.

Answer 1

The Correct Option is C

Step 1: Understand the concept of colour in transition metal ions.
Colour in transition metal compounds arises due to $d$–$d$ electronic transitions. Ions with completely filled or empty $d$ orbitals generally form colourless compounds.
Step 2: Analyze each option.
(A) $Sc^{3+$:} Electronic configuration is $3d^0$. No $d$–$d$ transition occurs, so compounds are colourless.
(B) $Cu^{+$:} Electronic configuration is $3d^{10}$. Fully filled $d$ orbitals form colourless compounds.
(C) $Ti^{3+$:} Electronic configuration is $3d^1$. Presence of a partially filled $d$ orbital allows $d$–$d$ transitions, producing coloured compounds.
(D) $Zn^{2+$:} Electronic configuration is $3d^{10}$. Fully filled $d$ orbitals result in colourless compounds.
Step 3: Conclusion.
$Ti^{3+}$ does not form colourless compounds because it has a partially filled $d$ orbital.