Question

Of the ions $Z{{n}^{2+}}$ , $N{{i}^{2+}}$ and $C{{r}^{3+}}$ [atomic number of $Zn=30,\text{ }Ni=28,\text{ }Cr=24$ ].
A.Only $Z{{n}^{2+}}$ is colourless and $N{{i}^{2+}}$and $C{{r}^{3+}}$are coloured.
B.All three are colourless.
C.All three are coloured.
D.Only $N{{i}^{2+}}$ is coloured and $Z{{n}^{2+}}$ and $C{{r}^{3+}}$ are colourless.

Answer 1

We know that the transition metal is an element with partially filled d subshell atoms. They are often referred to as 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.

Complete answer:
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. $Z{{n}^{2+}}\text{ }3d$ is completely filled and they are no unpaired electrons available for transition, so the transitions take place from the different shell which is higher in energy and thus these transitions requires more energy than the energy provided by visible light, that is they absorb light from ultraviolet region and hence, they appear white.
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. $N{{i}^{2+}}$ and $C{{r}^{3+}}$ are coloured due to the presence of unpaired d-orbitals electrons. But $Z{{n}^{2+}}$ is colourless because of the absence of unpaired electrons.
Therefore, the correct answer is option A.

Note:
Remember that the unpaired electron absorbs particular wavelengths in the visible region of light getting excited and the transmitted light shows the complementary colour which is blue. The striking colour shown by transition metals are caused by excitation of an electron from a lower-energy d orbital to a higher-energy d orbital. This phenomenon is called a d–d transition