GATE 2024 PH – Question 21
Atomic numbers of V, Cr, Fe and Zn are 23, 24, 26 and 30, respectively. Which one of the following materials does NOT show an electron spin resonance (ESR) spectra?
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Correct answer: (D) Zn
Explanation
**Electron spin resonance (ESR)** detects species with **unpaired electrons**. An atom or ion with all its electrons paired has no net spin and gives no ESR signal.
Electron configurations:
- **V** (Z = 23): $[\text{Ar}]3d^34s^2$, unpaired $d$ electrons. ESR active.
- **Cr** (Z = 24): $[\text{Ar}]3d^54s^1$, six unpaired electrons. ESR active.
- **Fe** (Z = 26): $[\text{Ar}]3d^64s^2$, unpaired $d$ electrons. ESR active.
- **Zn** (Z = 30): $[\text{Ar}]3d^{10}4s^2$, with **filled** $3d$ and $4s$ shells, so no unpaired electron. ESR inactive. ✓
Answer **Zn** (D).