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Answer the following.
Why Zn²⁺ ions are colourless while Ni²⁺ ions are green and Cu²⁺ ions are blue in colour?
Why $\text{Zn}^{2+}$ ions are colourless while $\text{Ni}^{2+}$ ions are green and $\text{Cu}^{2+}$ ions are blue in colour?
Answer
Answer
Colour arises from d-d transitions of unpaired electrons absorbing visible light.
$\mathrm{Zn^{2+}}$ ($3d^{10}$) has no unpaired electrons, so no d-d transition occurs and it is colourless.
$\mathrm{Ni^{2+}}$ ($3d^8$) and $\mathrm{Cu^{2+}}$ ($3d^9$) have unpaired electrons, so they absorb part of visible light and show the complementary colours, green and blue respectively.