Intermolecular Forces

Molecular Polarity

• In polar covalent bonds, electrons are shared unequally between two different non-metals
• To determine if a molecule is polar, we need to look at the whole molecule, and not just the individual bonds

1. If there are no polar bonds in the molecule, the molecule is non-polar! Example: O2

1. If there are polar bonds that are symmetrical, the dipole moments cancel each other out so the molecule is non-polar! Example: CO2

1. If there are polar bonds that are not symmetrical, the dipole moments don't cancel each other out and we are left with a net dipole moment, hence the molecule is polar! Example: H2O

Types of Intermolecular Forces

• Intermolecular forces are forces of attraction between molecules.
• Intermolecular forces define physical properties of compounds (boiling points, melting points, surface tension etc.)
• The stronger the intermolecular forces are, the higher the boiling and melting points will be

• The strongest of the intermolecular forces
• Hydrogen bonded to N, O, or F is attracted to lone pairs of electrons on other N, O or F

Dipole - Dipole

• Second strongest intermolecular force
• The more polar a molecule is, the stronger the forces
• Interaction between two polar molecules; opposite dipoles attract (δ– and δ+)

• The weakest of intermolecular forces.
• All molecules have London dispersion forces.
• The bigger the molecule, the stronger the forces
• Molecules interact randomly and distort each other's electron clouds, causing temporary dipoles
• Electrons randomly move around and at some point, more electrons may be on one side than the other in a non-polar molecule, creating a temporary dipole. This can happen with another non-polar molecule as well and the two can interact

How to Determine Molecular Polarity