Question
Out of $\mathrm{Cu}^{2+}, \mathrm{Fe}^{2+}$ and $\mathrm{Cr}^{3+}$, which ion is most paramagnetic and why?

Answer

Electronic configuration of:
$\mathrm{Cu}^{2+}=[\mathrm{Ar}] 3 \mathrm{~d}^9$
$\mathrm{Fe}^{2+}=[\mathrm{Ar}] 3 \mathrm{~d}^6$
$\mathrm{Cr}^{3+}=[\mathrm{Ar}] 3 \mathrm{~d}^3$
Paramagnetism depends on the number of unpaired electrons. From the electronic configuration of these ions, it is clear that $\mathrm{Cu}^{2+}$ has one, $\mathrm{Fe}^{2+}$ has four and $\mathrm{Cr}^{3+}$ has three unpaired electrons. Therefore, $\mathrm{Fe}^{2+}$ is most paramagnetic in nature.

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