Question

Colourless ion in the following is –

• A. Ni$^{2+}$
• B. Fe$^{3+}$
• C. Cu$^{2+}$
• D. Cu$^{+1}$

Hint:

Transition metal ions with either completely filled (d$^{10}$) or completely empty (d$^0$) d-orbitals are usually colourless.

Answer 1

The Correct Option is D

The colour of transition metal ions arises due to d–d electronic transitions in partially filled d-orbitals.

Step 1: Analyze each ion.
- (A) Ni$^{2+}$: Configuration = [Ar] 3d$^8$ $\Rightarrow$ partially filled d-orbitals $\Rightarrow$ coloured.
- (B) Fe$^{3+}$: Configuration = [Ar] 3d$^5$ $\Rightarrow$ half-filled d-orbitals $\Rightarrow$ coloured.
- (C) Cu$^{2+}$: Configuration = [Ar] 3d$^9$ $\Rightarrow$ partially filled d-orbitals $\Rightarrow$ coloured.
- (D) Cu$^{+1}$: Configuration = [Ar] 3d$^{10}$ $\Rightarrow$ completely filled d-orbitals, no d–d transitions possible $\Rightarrow$ colourless.
Step 2: Final Answer.
Hence, the colourless ion is Cu$^{+1}$.
\[ \boxed{\text{Cu}^{+1}} \]