Overview
This lesson introduces intermolecular forces, compares them with intramolecular forces, and explains the types and strengths of intermolecular forces using examples.
Intra- vs. Intermolecular Forces
- Intramolecular forces are forces within a molecule, such as covalent bonds that hold atoms together.
- Intermolecular forces are forces of attraction between different molecules, not actual bonds.
Types of Intermolecular Forces
- Dipole-dipole interactions occur between the positive side of one polar molecule and the negative side of another.
- Hydrogen bonding is a special, strongest type of dipole-dipole interaction where hydrogen is bonded to oxygen, nitrogen, or fluorine and attracted to these atoms in another molecule.
- London dispersion forces are weak, instantaneous attractions caused by temporary shifts in electrons, present in all molecules but dominant in nonpolar ones.
Strength Comparison of Intermolecular Forces
- Hydrogen bonding is the strongest intermolecular force.
- Dipole-dipole interactions are of intermediate strength.
- London dispersion forces are the weakest.
- All intermolecular forces are much weaker than intramolecular (bonding) forces.
Example Problems
- NO2⁻: Bent shape (VSEPR), polar molecule; exhibits London dispersion forces and dipole-dipole interactions but no hydrogen bonding.
- HNO: Bent shape (VSEPR), polar molecule; exhibits London dispersion, dipole-dipole, and hydrogen bonding due to H bonded to N/O/F.
- CH4: Tetrahedral shape (VSEPR), nonpolar molecule; only London dispersion forces present.
Key Terms & Definitions
- Intramolecular Forces — Forces holding atoms together within a molecule (e.g., covalent bonds).
- Intermolecular Forces — Forces of attraction between molecules.
- Dipole-Dipole Interaction — Attraction between polar molecules.
- Hydrogen Bonding — Strongest dipole-dipole force involving H bonded to O, N, or F.
- London Dispersion Forces — Weakest, temporary attraction caused by shifting electrons.
- VSEPR (Valence Shell Electron Pair Repulsion) — Model to predict molecular shape based on electron groups.
Action Items / Next Steps
- Practice drawing Lewis dot structures and VSEPR diagrams for assigned molecules.
- Prepare for next lesson on physical properties of ionic and covalent compounds.