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Answer the following.
Why are Zn²⁺ ions colourless while Ni²⁺ ions are green in colour?
Why are $\text{Zn}^{2+}$ ions colourless while $\text{Ni}^{2+}$ ions are green in colour?
Answer
Answer
$\mathrm{Zn^{2+}}$ ($3d^{10}$) has no unpaired electrons, so no $d$-$d$ transition occurs and it is colourless. $\mathrm{Ni^{2+}}$ ($3d^8$) has unpaired electrons, so $d$-$d$ transitions absorb part of visible light and it appears green.