Attractions Between Molecules
While intramolecular forces are the strong forces that hold atoms together within a molecule (i.e., covalent and ionic bonds), intermolecular forces (IMFs) are the much weaker attractive forces that exist between molecules. IMFs are responsible for determining a substance's physical state (solid, liquid, or gas) and properties like boiling point and viscosity.
Types of Intermolecular Forces (from weakest to strongest)
1. London Dispersion Forces (LDF)
Present in: All atoms and molecules. It is the only IMF present in nonpolar molecules.
Mechanism: Caused by the constant, random motion of electrons. At any given instant, the electron cloud can be temporarily unevenly distributed, creating a temporary, instantaneous dipole. This temporary dipole can then induce a similar dipole in a neighboring molecule, leading to a weak, fleeting attraction.
Strength: Increases with the size of the molecule (more electrons = more 'slosh' = more polarizable).
2. Dipole-Dipole Forces
Present in: Polar molecules only.
Mechanism: Polar molecules have permanent positive and negative ends (a permanent dipole). The dipole-dipole force is the electrostatic attraction between the partial positive end (δ⁺) of one molecule and the partial negative end (δ⁻) of a neighboring molecule.
Strength: Generally stronger than LDFs for molecules of similar size.
3. Hydrogen Bonds
Present in: Molecules where a hydrogen atom is covalently bonded to a highly electronegative atom, specifically Nitrogen (N), Oxygen (O), or Fluorine (F).
Mechanism: This is an especially strong type of dipole-dipole interaction. The H-N, H-O, or H-F bond is extremely polar, leaving the hydrogen atom with a strong partial positive charge. This highly exposed proton is then strongly attracted to the lone pair of electrons on a nearby N, O, or F atom.
Strength: The strongest type of intermolecular force. It is responsible for the unique properties of water, such as its unusually high boiling point.
IMFs and Physical Properties:
Stronger IMFs → Higher Boiling Point and Melting Point. More energy is required to overcome the attractions between molecules to allow them to escape into the gas phase (boil) or move freely as a liquid (melt).