Intermolecular Forces Notes

Intramolecular forces = forces WITHIN a single molecule (i.e., Covalent or Ionic Bonds)

Intermolecular forces = forces between molecules that result in physical properties of substances

The state of matter (solid,liquid,gas) are a result of intermolecular forces. Study the following chart:

Intermolecular Forces (IMF) found in Various States of Matter

State of Matter / Intermolecular Forces / Effect caused by IMF
Solids / Very Strong /
  • Atoms are held close together and in place
  • Cannot be compressed
  • Do not flow (like water or gases do)
  • Have high melting and boiling points

Liquids / Moderate /
  • Molecules are held closer together but can move around and slide past each other
  • Can flow and be poured, but are not compressible (there is no extra space between the molecules)
  • Have moderately high melting/boiling points

Gases / Very weak /
  • Molecules can spread out and move about
  • Have no definite shape or value and are highly compressible because there is so much room between them
  • Have low meting and boiling points

NOTICE from above…weakIMFs = low melting/boiling points,

strong IMFs = high melting/boiling points!

Intermolecular Forces we will study:

  1. London dispersion forces
  2. Dipole to dipole forces
  3. Hydrogen bonding
  4. Ion-dipole forces
  1. London dispersionforces

Forces of attraction between molecules based on motion of electrons, these are the only IMFs in nonpolar molecules but can and do exist in any type

Larger molecules, molecules with higher masses have more electrons and are more polarizable (can move electrons around to get an instantaneous dipole)

The more polarizable, the stronger the London forces

  1. Therefore, the bigger/heavier the molecule (the more electrons), the stronger the London forces
  1. Dipole-dipole forces

Forces of attraction between polar molecules (between positive and negative ends of different molecules)

The strength of these forces increases with increasing polarity

The more polar, the stronger the dipole forces

  1. Hydrogen bond forces

Special type of dipole-dipole force between H atoms in a polar bond and a small, electronegative atom (F, O, or N)

This strong forces causes the physical properties of substances to be high(ex: abnormally high boiling point of water, ammonia, and hydrogen fluoride compared to other compounds of elements)

  1. Ion-dipole forces

Force between an ion and the partial charge on the end of a polar molecule

Exist in aqueous solutions (this is what helps water pull apart the ions in an ionic compound when they dissolve)

How to determine what Intermolecular Force may be between substances:

• Identifying type:

Everything has at least London disperson

If the molecule is nonpolar- has London dispersion forces only

If the molecule is polar – also has dipole-dipole

If H bonded to F, O, or N – also has hydrogen bonding

If an ionic compound in aqueous solution – ion-dipole (we won’t use too many of these)

Comparison of strength:

Hydrogen bonding forces – strongest

Diple to dipole forces – weaker than hydrogen but stronger than London

London Dispersion forces: weakest

PLEASE NOTE: ALL INTERMOLECULAR FORCES ARE WEAKER THAN ACTUAL INTRAMOLECULAR BONDS (ionic, covalent and metallic).

• If you are asked to rank substances based on their melting or boiling points, remember… the stronger the IMFs, the higher the melting and boiling points because it is harder to separate the molecules from each other!