Intermolecular forces

intermolecular force aka…

van der waals force

What are intermolecular force?

an attraction between molecules

What are the 3 intermolecular forces?

dipole-dipole forces, hydrogen bonding, and dispersion (london dispersion force)

Describe dipole-dipole

• electrostatic forces of attraction between 2 polar molecules 

• partial negative charge on the side of the molecule will be attracted to the partial positive change on a neighboring molecules creating a weak bond

• polar molecules created when at least one of the covalent bonds in a molecule has an ∆EN of greater than 0.4 and the shape of the molecule is unbalanced 

• the forces of attraction is weaker due to only being partial charge and the distance involved being greater than the covalent bond

Describe hydrogen bonding

• particularly strong versions of dipole-dipole force (strongest intermolecular force)

• occurs only when H is directly bonded to: N or O or F

• this creates a partial positive charge on the H which will be attracted to a partial negative charge on the N/O/F

  • e.g. H₂O and NH₃

Describe dispersion (london dispersion force)

• all covalent molecules can experience dispersion forces

• BUT they usually only become a factor in the properties of non-polar molecules 

• the random motion of the e^- in the atom can result in one side of the atom/ molecules having a slight negative charge as e^- gather together

• there is a very weak electrostatic force of attraction between the partial positive and partial negative charge that is extremely temporary as the random motion of the e^-  will cause the bond to break instantly 

• the sum of all these very weak and temporary forces is what holds non-polar bonds together