Molecular Orbital Theory (MOT) Mnemonics; Easy Memory Tricks
1. Order of Molecular Orbitals (σ & π)
For diatomic molecules (B₂ to N₂) (s-p mixing changes order):
Mnemonic: “BON” (B₂, O₂, N₂ have different orders)
- For B₂, C₂, N₂: σ(2s) < σ(2s) < π(2pₓ) = π(2pᵧ) < σ(2pz) < π(2pₓ) = π(2pᵧ) < σ(2pz)
- For O₂, F₂, Ne₂: σ(2s) < σ(2s) < σ(2pz) < π(2pₓ) = π(2pᵧ) < π(2pₓ) = π(2pᵧ) < σ(2pz)
Mnemonic:
- “Be Careful, Nitrogen!” (B₂, C₂, N₂ have π before σ)
- “Oxygen Fights Neon!” (O₂, F₂, Ne₂ have σ before π)
2. Bond Order Formula
Bond Order = (Bonding e⁻ – Antibonding e⁻) / 2
Mnemonic: “BOA” (Bond Order = Bonding – Antibonding)
3. Magnetic Nature
- Paramagnetic (unpaired e⁻) → Attracted by magnetic field (e.g., O₂)
- Diamagnetic (all e⁻ paired) → Repelled by magnetic field (e.g., N₂)
Mnemonic:
- “Para Pulls, Dia Dismisses”
- “O₂ is Odd” (O₂ has 2 unpaired e⁻ → paramagnetic)
4. Stability & Bond Order
- Higher Bond Order = Stronger & Shorter Bond
- Bond Order = 0 → No bond (e.g., He₂)
Mnemonic: “Bigger BO, Stronger Glue”
5. HOMO & LUMO
- HOMO (Highest Occupied MO) → Most reactive e⁻
- LUMO (Lowest Unoccupied MO) → Accepts e⁻
Mnemonic: “HOMO is Home, LUMO is Empty”
