Question

Which element from the following forms colourless compounds in +2 oxidation state?

• A. Co (Z = 27)
• B. Zn (Z = 30)
• C. Cu (Z = 29)
• D. Mn (Z = 25)

Hint:

Elements with completely filled d-orbitals like \(\text{Zn}^{2+}\), \(\text{Cd}^{2+}\), and \(\text{Hg}^{2+}\) generally form colourless compounds.

Answer 1

The Correct Option is B

Step 1: Understanding the concept.
The colour of compounds formed by transition elements mainly depends on the presence of partially filled d-orbitals. If the d-orbitals are completely filled or completely empty, no d–d electronic transitions occur, resulting in colourless compounds.

Step 2: Electronic configuration analysis.
Zinc (Z = 30) has the electronic configuration \([Ar]\,3d^{10}4s^2\). In the +2 oxidation state, zinc forms \(\text{Zn}^{2+}\) ions with configuration \([Ar]\,3d^{10}\), which has a completely filled d-subshell.

Step 3: Analysis of options.
(A) Co (Z = 27): \(\text{Co}^{2+}\) has partially filled d-orbitals, so its compounds are coloured.
(B) Zn (Z = 30): Correct — \(\text{Zn}^{2+}\) has a completely filled d-subshell, so its compounds are colourless.
(C) Cu (Z = 29): \(\text{Cu}^{2+}\) has an incomplete d-subshell and forms coloured compounds.
(D) Mn (Z = 25): \(\text{Mn}^{2+}\) has half-filled d-orbitals and forms coloured compounds.

Step 4: Conclusion.
Zinc forms colourless compounds in the +2 oxidation state due to the absence of d–d transitions.