The metal ions that have the calculated spin only magnetic moment value of 4.9 B.M. are A. $\mathrm{Cr}^{2+}$ B. $\mathrm{Fe}^{2+}$ C. $\mathrm{Fe}^{3+}$ D. $\mathrm{Co}^{2+}$ E. $\mathrm{Mn}^{3+}$ Choose the correct answer from the options given below

JEE Main · 2025 · 03 Apr 2025 · Shift 1 · d and f Block Elements

The metal ions that have the calculated spin only magnetic moment value of 4.9 B.M. are
A.

Cr^{2 +}

Fe^{2 +}

Fe^{3 +}

Co^{2 +}

Mn^{3 +}

Choose the correct answer from the options given below

Answer: (D)

A spin-only moment of

4.9

B.M. satisfies

μ = n (n + 2)

with

n (n + 2) \approx 24

, giving

n = 4

unpaired electrons. Counting d-electrons:

Cr^{2 +} (3 d^{4})

has 4,

Fe^{2 +} (3 d^{6})

has 4 in the high-spin state,

Fe^{3 +} (3 d^{5})

has 5,

Co^{2 +} (3 d^{7})

has 3 and

Mn^{3 +} (3 d^{4})

has 4. The ions with exactly four unpaired electrons, and hence

μ = 4.9

B.M., are

Cr^{2 +}

Fe^{2 +}

and

Mn^{3 +}

JEE Main · 2025 · 03 Apr 2025 · Shift 1 · Single correct option