Q12 / 75 · JEE Main 2025 (8 Apr Shift 2) · Chemistry

The correct decreasing order of spin only magnetic moment values $(\mathrm{BM})$ of $\mathrm{Cu}^{+}, \mathrm{Cu}^{2+}, \mathrm{Cr}^{2+}$ and $\mathrm{Cr}^{3+}$ ions is :

  • A
    $\mathrm{Cu}^{+} \gt \mathrm{Cu}^{2+} \gt \mathrm{Cr}^{3+} \gt \mathrm{Cr}^{2+}$
  • B
    $\mathrm{Cu}^{2+} \gt \mathrm{Cu}^{+} \gt \mathrm{Cr}^{2+} \gt \mathrm{Cr}^{3+}$
  • Correct answer
    C
    $\mathrm{Cr}^{2+} \gt \mathrm{Cr}^{3+} \gt \mathrm{Cu}^{2+} \gt \mathrm{Cu}^{+}$
  • D
    $\mathrm{Cr}^{3+} \gt \mathrm{Cr}^{2+} \gt \mathrm{Cu}^{+} \gt \mathrm{Cu}^{2+}$

Solution

$\mathrm{Cu}^{+}:[\mathrm{Ar}] 3 \mathrm{~d}^{10}$, Spin only magnetic moment $=0$
B.M.
$\mathrm{Cu}^{+2}:[\mathrm{Ar}] 3 \mathrm{~d}^9$, Spin only magnetic moment $=\sqrt{3}$
B.M.
$\mathrm{Cr}^{+2}:[\mathrm{Ar}] 3 \mathrm{~d}^4$, Spin only magnetic moment $=\sqrt{24}$
B.M.
$\mathrm{Cr}^{+3}:[\mathrm{Ar}] 3 \mathrm{~d}^3$, Spin only magnetic moment $=\sqrt{15}$
B.M.
Order of $\mu$ : $\mathrm{Cr}^{+2} \gt \mathrm{Cr}^{+3} \gt \mathrm{Cu}^{+2} \gt \mathrm{Cu}^{+}$

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

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