Question

For the ions $Zn^{2+}, Ni^{2+}$ and $Cr^{3+}$ which among the following statements is correct?
(atomic number of $Zn = 30, Ni = 28$ and $Cr = 24)$

• A. All these are colourless
• B. All these are coloured
• C. Only $Ni^{2+}$ is coloured and $Zn^{2+}$ and $Cr^{3+}$ are colourless
• D. Only $Zn^{2+}$ is colourless and $Ni^{2+}$ and $Cr^{3+}$ are coloured

Answer 1

Answer: (D)

The ions with unpaired electrons are coloured and those with paired electrons are colourless.
$\underset{\text{No. of } e^-s = 28}{Zn^{2+}} = 1s^2, 2s^2p^6, 3s^2p^6d^{10}$
$\underset{\text{No. of } e^-s = 21}{Cr^{3+}} = 1s^2, 2s^2p^6, 3s^2p^6d^{3}$
$\underset{\text { (No. of } \left.e^{-} s=26\right)}{ Ni ^{2+}}=1 s^2, 2 s^2 p^6, 3 s^2 p^6 d^8$
Thus $Zn ^{2+}, Cr ^{3+}$ and $Ni ^{2+}$ have zero, $3$ and $2$ unpaired electrons respectively.