Inter molecular Forces

• Intermolecular forces


  Forces between molecules, not the forces inside a molecule (not covalent bonds)
• Van der Waals forces/London forces

  • Very weak forces
  • Caused by temporary dipoles in atoms due to uneven distribution of electrons
• Permanent dipole-dipole forces


  • Molecules have a permanent positive and negative end
  • Positive and negative ends of different molecules attract each other
• Hydrogen bonds


  • Special case of permanent dipole-dipole forces
  • Occur when hydrogen is bonded to fluorine, oxygen or nitrogen
  • Create large dipoles, e.g. in water molecules
• If not for hydrogen bonds

  Water would be a gas at room temperature, and there would be no liquid water for life on Earth
• Relative strength of intermolecular bonds compared to covalent bonds (covalent bond = 100):