Question

Which of the following prediction is correct when the transition metal ion is colourless?

• A. it contains 0 unpaired electrons
• B. it contains 1 unpaired electrons
• C. it contains 2 unpaired electrons
• D. it contains 3 unpaired electrons

Hint:

Colour in transition ions comes from d-d transitions, which need unpaired d electrons.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept
Transition metal ions are coloured because an electron can jump between split d orbitals by absorbing visible light (a d-d transition). For this jump an orbital in the d set must be able to take a paired or unpaired electron from a lower one.

Step 2: Link to unpaired electrons
If the d subshell is empty (d\(^0\)) or completely full (d\(^{10}\)), no d-d transition can occur. Such ions have zero unpaired electrons and appear colourless, for example \(\text{Sc}^{3+}\) (d\(^0\)) and \(\text{Zn}^{2+}\) (d\(^{10}\)).

Step 3: Check the options
(A) 0 unpaired electrons: consistent with a colourless ion. (B), (C) and (D) all have unpaired electrons, which usually leads to colour.

Step 4: Conclusion
The correct prediction is zero unpaired electrons, option (A).

Final Answer:
A colourless transition metal ion has no unpaired d electrons, option (A). \[ \boxed{\text{0 unpaired electrons}} \]