Magnetic moment of \(Co^{2+}\) will be
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 3.87 BM
Cobalt has atomic number 27:
Co: [Ar] 3d7 4s2 ⇒ \( \ce{Co^{2+}} \): [Ar] 3d7 (the two 4s electrons are lost first)
The seven 3d electrons fill the five d orbitals singly first (Hund’s rule) and then pair up: ↑↓ ↑↓ ↑ ↑ ↑, so there are n = 3 unpaired electrons. The spin-only magnetic moment is
\( \mu = \sqrt{n(n+2)}\ \text{BM} = \sqrt{3 \times 5}\ \text{BM} = \sqrt{15}\ \text{BM} = 3.87\ \text{BM} \)
NCERT (2026-27 edition):
Magnetic Properties – …
