Question

For ${Zn^{2+}}$, ${Ni^{2+}}$, $Cu$, and ${Cr^{2+}}$ which of the following statements is correct?
A. Only ${Zn^{2+}}$ is colourless and ${Ni^{2+}}$, ${Cu^{2+}}$ and Cr are coloured
B. All the ions are coloured
C. All the ions are colourless
D. ${Zn^{2+}}$ and ${Cu^{2+}}$ are colourless while ${Ni^{2+}}$ and ${Cr^{2+}}$ are coloured

Answer 1

Hint: All these given elements are all transition elements. These elements have partially filled d orbitals. They are known to form stable cations with their incompletely filled d-orbital.

Complete answer:
- A transition metal is an element with partially filled d subshell atoms. They are often referred to to the d block elements.
- One of their exceptional properties is their colour. It is proven that some visible spectra are absorbed by these elements from white light as it passes through a sample of transition metals.
- When an electron jumps from lower energy d orbital to higher energy d orbital, that's a d-d transition, the energy of excitation corresponds to the frequency of sunlight absorbed.
- Therefore, excitement of an electron from lower energy level to higher energy level requires more energy. Therefore, not all transition metal complexes are coloured because transition elements with fully filled d orbital don't allow the probability of d-d transitions. Hence, no radiations are absorbed.
- Since zinc has a fully filled d-orbital without any unpaired electron, it does not undergo electronic transition. Hence, zinc is colourless.
- In the case of nickel, copper, and chromium, they all have unpaired electrons which suggest they all undergo transition. And so, they are coloured.

- Therefore, the answer to the question is (A)

Note: The striking colours shown by transition metals are caused by excitation of an electron from a lower-energy of d orbital to a higher-energy of d orbital. This phenomenon is called a d–d transition