Aufbau Principle, Hund’s Rule & Pauli Exclusion Principle
Visualizing electron filling with spin arrows
Interactive Orbital Filling
Click Add Electron to see electrons fill orbitals with spins (↑ or ↓), following the Aufbau Principle, Hund’s Rule, and Pauli Exclusion Principle.
1. Aufbau Principle
The Aufbau Principle says electrons occupy the lowest energy orbital first. Example: Oxygen (Z = 8) has configuration 1s² 2s² 2p⁴.
2. Hund’s Rule
Hund’s Rule: Electrons occupy orbitals singly with parallel spins before pairing. This minimizes repulsion and gives stability.
3. Pauli Exclusion Principle
Pauli Principle: No two electrons in the same atom have identical quantum numbers. Each orbital holds max 2 electrons with opposite spins (↑↓).
✨ Exceptions in Aufbau Principle: Chromium (Cr) and Copper (Cu)
Normally, electrons fill orbitals according to the Aufbau Principle — i.e., in increasing order of energy levels (1s → 2s → 2p → 3s → 3p → 4s → 3d...). But, Chromium (Z=24) and Copper (Z=29) are important exceptions.
🔹 Chromium (Cr)
Expected Configuration: [Ar] 3d4 4s2
Observed Configuration: [Ar] 3d5 4s1
✔ Reason: Half-filled 3d (d5) orbitals gain extra stability due to maximum exchange energy.
Expected (3d4 4s2)
Observed (3d5 4s1)
🔹 Copper (Cu)
Expected Configuration: [Ar] 3d9 4s2
Observed Configuration: [Ar] 3d10 4s1
✔ Reason: Completely filled 3d (d10) orbitals are highly stable due to symmetrical distribution.
Expected (3d9 4s2)
Observed (3d10 4s1)
✅ Conclusion
Chromium prefers half-filled 3d (d5) and Copper prefers completely filled 3d (d10), because such arrangements give extra stability due to exchange energy and symmetry. Hence, they deviate from the normal Aufbau filling order.
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