Which of the following has the maximum number of unpaired ' $d$ ' electrons?

• A

  $\mathrm{Zn}^{2+}$
• Correct answer
  B

  $\mathrm{Fe}^{2+}$
• C

  $\mathrm{Ni}^{3+}$
• D

  $\mathrm{Cu}^{+}$

Solution

$$ \begin{aligned} \mathrm{Zn}(30)=& {[\mathrm{Ar}] 3 \mathrm{~d}^{10}, 4 \mathrm{~s}^{2} } \\ \mathrm{Zn}^{2+} &=[\mathrm{Ar}] 3 \mathrm{~d}^{10} \text { (no unpaired electron) } \\ \mathrm{Fe}(26) &=[\mathrm{Ar}] 3 \mathrm{~d}^{6}, 4 \mathrm{~s}^{2} \\ \mathrm{Fe}^{2+} &=[\mathrm{Ar}] 3 \mathrm{~d}^{6} \text { (four unpaired electrons) } \end{aligned} $$

$\begin{aligned} \mathrm{Ni}(28) &=[\mathrm{Ar}] 3 \mathrm{~d}^{8}, 4 \mathrm{~s}^{2} \\ \mathrm{Ni}^{3+} &=[\mathrm{Ar}] 3 \mathrm{~d}^{7} \text { (three unpaired electrons) } \end{aligned}$

$$ \begin{aligned} \mathrm{Cu}(29) &=[\mathrm{Ar}] 3 \mathrm{~d}^{10}, 4 \mathrm{~s}^{1} \\ \mathrm{Cu}^{+} &=[\mathrm{Ar}] 3 \mathrm{~d}^{10} \text { (no unpaired electron) } \end{aligned} $$

Hence, the correct answer is option: 'B'.