Overview
This lecture explains the difference between intramolecular and intermolecular forces, outlines their types, strengths, and roles in determining physical properties of substances.
Intramolecular vs Intermolecular Forces
- Intramolecular forces hold atoms together within a molecule (e.g., bonds inside HCl).
- Intermolecular forces exist between separate molecules (e.g., attractions between HCl molecules).
- Intramolecular forces are much stronger than intermolecular forces.
Types of Intramolecular Forces
- Ionic bond: Complete transfer of electrons from metal (cation) to nonmetal (anion).
- Covalent bond: Atoms share electrons due to similar electronegativities; can be nonpolar (<0.5 difference) or polar (0.5–1.9 difference).
- Metallic bonding: Valence electrons are delocalized and move freely among metal ions.
- Relative strength: Metallic > Ionic > Polar covalent > Nonpolar covalent.
Types of Intermolecular Forces
- Dipole-dipole interactions: Attraction between partial positive and negative charges of polar molecules; strongest general intermolecular force.
- Hydrogen bonding: Special dipole-dipole interaction between H and O, N, or F; strongest of dipole-dipole, important in water, HF, DNA.
- London dispersion forces: Weak attractions due to temporary dipoles; present in all molecules, strength increases with more electrons.
Effects on Physical Properties
- Stronger intermolecular forces lead to higher boiling/melting points and higher enthalpy of fusion and vaporization.
- Ionic compounds have highest boiling/melting points, followed by hydrogen-bonded covalent, then polar covalent, and lastly nonpolar covalent compounds.
Examples of Intermolecular Forces in Compounds
- H₂S: London dispersion and dipole-dipole forces.
- CH₃OH: London dispersion, dipole-dipole, and hydrogen bonding.
- C₂H₆: Only London dispersion forces.
Key Terms & Definitions
- Intramolecular force — Force holding atoms together within a molecule.
- Intermolecular force — Force between separate molecules.
- Ionic bond — Bond from complete electron transfer, creating ions.
- Covalent bond — Bond from electron sharing; polar or nonpolar.
- Metallic bond — Attraction between metal ions and free electrons.
- Dipole-dipole interaction — Forces between polar molecules.
- Hydrogen bond — Strong dipole-dipole force between H and O, N, or F.
- London dispersion force — Weak force from temporary dipoles in all molecules.
Action Items / Next Steps
- Review definitions and characteristics of all force types.
- Practice identifying force types present in various compounds.
- Study the effect of different forces on melting and boiling points for exam prep.