Electron Configuration Rules: Aufbau, Pauli & Hund’s Mnemonics
1. Aufbau Principle (Electron Filling Order)
Mnemonic: “Smart People Don’t Fail”
- S (1s)
- P (2s 2p)
- D (3s 3p 3d)
- F (4s 4p 4d 4f)
Alternative (Diagonal Rule):
“1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s…”
(Remember: “Lowest energy first!”)
2. Pauli Exclusion Principle
Mnemonic: “Opposites Attract in the Same Seat”
- Each orbital holds max 2 electrons with opposite spins (↑↓).
- Visualize: A couple sitting on a bench facing opposite directions.
3. Hund’s Rule (Orbital Filling)
Mnemonic: “Empty Buses Fill Seats One at a Time”
- Electrons occupy empty orbitals singly first (↑ ↑ ↑) before pairing (↑↓).
- Analogy: People prefer sitting alone on a bus rather than sharing seats immediately.
Combined Quick Reference:
“Fill Smart (Aufbau), No Twins (Pauli), Spread Out First (Hund).”
Common Exceptions:
- Cr, Cu: Prefer half-full/full d-orbitals (e.g., Cr = [Ar] 3d⁵ 4s¹).
- Ag, Au: Similar exceptions in 4d/5d series.
Why These Work Better:
Shorter and more intuitive than traditional mnemonics. Action-based language (“Fill,” “Spread Out”) reinforces the rules.
Clear visuals (bus seats, couples) aid recall.
