Dipole Moment & Bond Polarity Mnemonics – Easy Tricks to Determine Molecular Polarity
1. Bond Polarity Basics
“EN Difference = Dipole Direction”
- Greater electronegativity (EN) difference → more polar bond
- Arrow points toward the more electronegative atom (δ⁻)
Mnemonic: “The Hungry Atom” (electronegative atom “pulls” electrons toward itself)
2. Dipole Moment (μ) Formula
μ = Q × r
(charge × distance between charges)
Mnemonic: “Queen Runs” (Q × r)
3. Molecular Polarity Rules
“Shape + Dipoles = Net Polarity”
- Polar bonds + asymmetrical shape = polar molecule (H₂O)
- Polar bonds + symmetrical shape = nonpolar molecule (CO₂)
Mnemonic: “Asymmetry Attacks!” (asymmetrical molecules with polar bonds are polar)
4. Common Polar/Nonpolar Patterns
| Pattern | Example | Mnemonic |
|---|---|---|
| Lone pair + polar bonds | H₂O, NH₃ | “Lonely and Polar” |
| Symmetrical dipoles cancel | CO₂, CCl₄ | “Symmetry Saves” |
| Single different terminal atom | CH₃Cl | “Odd One Out” |
5. Quick Polarity Checklist
- Are there polar bonds? (EN difference > 0.4)
- Is molecular shape asymmetrical?
- Do dipoles cancel?
Mnemonic: “PAS” (Polar bonds? Asymmetrical? Sum of dipoles)
6. Special Cases
- Hydrocarbons (C-H bonds): Nearly nonpolar (EN difference 0.4)
- O-H, N-H bonds: Always polar (big EN difference)
- Diatomics (O₂, N₂): Nonpolar (same atom)
Mnemonic: “OHN” (O-H/N-H = Always polar, Others check PAS)
7. Dipole Moments in Exam Questions
When asked “Which has greater dipole moment?”:
- Compare bond polarity (EN difference)
- Compare molecular asymmetry
- Remember: Bent > Trigonal Pyramidal > Tetrahedral
Mnemonic: “Bent is Biggest” (H₂O > NH₃ > CH₄)
Bonus: Organic Chemistry Polarity
- Alcohols (R-OH): Polar (due to O-H)
- Ethers (R-O-R’): Slightly polar
- Alkanes (C-C/C-H): Nonpolar
Mnemonic: “Alcohol Always Polar” (AAP)
