Quantum Numbers

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Number of unpaired electrons in Cu2+ (Z=29) is:

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1. **Neutral Copper ($Z=29$)**: Configuration is an exception to Aufbau: $[Ar] 3d^{10} 4s^1$.
2. **Ionization Rules**: Electrons are first removed from the shell with the highest principal quantum number ($n$).
3. **Forming $Cu^{2+}$**:
   - Remove $1$ electron from $4s$: yields $Cu^+ ([Ar] 3d^{10})$.
   - Remove $1$ electron from $3d$: yields $Cu^{2+} ([Ar] 3d^9)$.
4. **Orbital Analysis**: The $d^9$ configuration consists of $5$ orbitals. Following Hund's rule, $4$ orbitals are fully paired and $1$ orbital has a **single unpaired electron**.
5. **Conclusion**: Copper(II) ions are paramagnetic due to this **one unpaired electron**.

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