Question

A colourless ion in the following is

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

Hint:

Transition metal ions with completely filled $d^{10}$ or empty $d^0$ configurations are colourless.

Answer 1

The Correct Option is A

Step 1: Recall concept.
Colour in transition metal ions arises due to $d$-$d$ electronic transitions in partially filled $d$ orbitals.
Step 2: Check ions.
- Cu$^+$ has configuration $[Ar] 3d^{10}$, fully filled $d$-orbitals $\Rightarrow$ no $d$-$d$ transitions, hence colourless.
- Cu$^{2+}$: $3d^9$, coloured.
- Ni$^{2+}$: $3d^8$, coloured.
- Fe$^{3+}$: $3d^5$, coloured.
Step 3: Conclude.
Only Cu$^+$ is colourless. Final Answer: \[ \boxed{Cu^+} \] % Quciktip