Which of the following transition metal ions has the highest magnetic moment in aqueous solution?

Solution

1. The electronic configuration of ${\text{Mn}}^{2+}$ is $3d^5$, which has $5$ unpaired electrons.
2. The electronic configuration of ${\text{Cr}}^{2+}$ is $3d^4$ ($4$ unpaired), ${\text{Fe}}^{2+}$ is $3d^6$ ($4$ unpaired), and ${\text{Co}}^{2+}$ is $3d^7$ ($3$ unpaired).
3. Magnetic moment $\mu =\sqrt{n(n+2)}$ Bohr magnetons, where $n$ is the number of unpaired electrons.
4. Since ${\text{Mn}}^{2+}$ has the maximum number of unpaired electrons, it has the highest magnetic moment.
5. Hence the answer is (B).