Intermolecular Forces

Shapes of Molecules

There are 5 basic shapes that you should get used to:

• Linear
  • E.g. HCl
• V-shaped
  • E.g. H2O
• Trigonal Planar
  • E.g. CH20 (Methanal)
• Pyramidal
  • E.g. NH3 (Ammonia)
• Tetrahedral
  • E.g. CH4 (Methane)

* Note: White circles represent a different atom to the black circles

This is due to the Valence Shell Electron Pair Repulsion Theory (VSEPR)

• Regions of charge distribute themselves around an atom by repulsion to maximise the distance from each other.

Non-Polar & Polar Bonds

Non-Polar Bond

• Even distribution of charge
• Electrons are shared equally - electrons are “in the middle”

Polar Bond

• E.g. Fluorine has a much higher electronegativity
• Electrons are attracted more to it
• There is an uneven charge distribution
• ∴ Polar Bond

Polarity of Molecules

To be polar:

1. There must be at least one bond that is polar
2. Molecule must be non-symmetric

Shape	Molecule	Diagram	Polar or Non-Polar	Why?
Tetrahedral	CH4		Non-Polar	Symmetric
Linear	CO2		Non-Polar	Symmetric
Any Same Molecule	H2		Non-Polar	Symmetric
Linear	HCl		Polar	Asymmetric
Trigonal Planar	BCl3		Non-Polar	Symmetric
V-Shaped	H2O		Polar	Asymmetric
Pyramidal	NH3		Polar	Asymmetric

Van der Waals Forces

• These are Intermolecular Forces between molecules
• The strength determines the M.P, B.P and other properties

1. Dipole - Dipole Attraction

• These affect polar molecules only
• Due to slight differences in charges on each side of the molecule

2. Dispersion Forces

• It is the only way non-polar molecules interact wich each other

1. Instantaneous dipole caused by random movement of electrons
2. Induce dipoles in neighboring molecules
3. Sets up dispersion force between molecules - The strength of the dispersion forces are determined by:
   • Number of electrons in the molecule
     • A larger number of electrons means temporary dipoles are larger and more frequent
   • Shape of molecules
     • Longer and Thinner is better

Polar Molecules can experience both dipole - dipole AND dispersion forces.