Intermolecular Forces

Intermolecular forces are the forces that hold molecules together (like in liquids and solids)

Dipole-Dipole Forces
  • Polar molecules have a positively-charged portion and a negatively-charged portion.
  • The oxygen of water molecules carries a slightly negative charge, and the hydrogen atoms carry a slightly positive charge.
  • The negatively-charged oxygen is attracted to the positively-charged hydrogens of other water molecules, and so the molecules stick together more.
  • More polar molecules → stronger dipole-dipole forces.

London Dispersion Forces
  • The electrons in some molecules will inevitably be attracted to the protons inside of other molecules.
  • If the electron cloud inside/around each molecule can be distorted, then London Dispersion Forces will play a stronger role.
  • The distort-ability of the electron cloud is measured as polarizability. Large atoms and large molecules are more polarizable.

Hydrogen Bonding
  • When hydrogen atoms are bonded to electronegative atoms like N, O, F or Cl, the hydrogen is left with a significantly positive charge (after all, the other atom “stole” its electrons!)
  • These hydrogen atoms bond with lone pairs of electrons on the N, O, F or Cl atoms of other molecules, and this attraction is very strong.
  • This is the strongest intermolecular force

Note: Covalent and ionic bonds are not intermolecular forces.  They are the forces that hold atoms together to form molecules/crystals, and are called inTRAmolecular.