Intermolecular forces (IMFs) are the forces of attraction between molecules, influencing physical properties like melting and boiling points. The main types of intermolecular forces are:
- London Dispersion Forces (LDF): Present in all molecules, polar or nonpolar. These arise from temporary fluctuations in electron distribution creating instantaneous dipoles that induce dipoles in neighboring molecules. LDF strength increases with molecular size and polarizability
- Dipole-Dipole Interactions: Occur between polar molecules with permanent dipoles. The partial positive end of one molecule attracts the partial negative end of another. These forces are stronger than LDF but weaker than hydrogen bonds
- Hydrogen Bonding: A special, strong type of dipole-dipole interaction occurring when hydrogen is covalently bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine. This results in strong attractions between molecules, significantly raising boiling and melting points
Summary of Intermolecular Forces by Compound Type
Compound Type| Intermolecular Forces Present| Notes
---|---|---
Nonpolar molecules| London dispersion forces only| Weakest IMF; present in all
molecules
Polar molecules (no H bonded to N, O, F)| London dispersion + dipole-dipole
interactions| Moderate strength
Molecules with H bonded to N, O, F| London dispersion + dipole-dipole +
hydrogen bonding| Strongest IMF among covalent molecules
Ionic compounds| Ion-ion interactions + London dispersion| Strongest overall
intermolecular forces
How to Identify IMFs in a Given Compound
- Check polarity:
- If nonpolar → only London dispersion forces.
- If polar → London dispersion + dipole-dipole.
- Check for H bonded to N, O, or F:
- If yes → hydrogen bonding present in addition to the above.
- If ionic compound:
- Ion-ion forces dominate, along with London dispersion.
Examples:
- Water (H₂O): Exhibits hydrogen bonding, dipole-dipole, and London dispersion forces
- Hydrogen chloride (HCl): Dipole-dipole and London dispersion forces
- Methane (CH₄): Only London dispersion forces (nonpolar molecule)
- Bromine (Br₂): London dispersion forces only, but stronger than chlorine due to more electrons
In summary, every compound exhibits London dispersion forces. Polar molecules also exhibit dipole-dipole forces, and molecules with H bonded to N, O, or F exhibit hydrogen bonding, the strongest of these intermolecular forces
