Intermolecular Forces

Forces between molecules
Allotropes: Different physical forms of the same element
- E.g., carbon, diamond, graphite
Gases: Very weak attraction between particles
Liquid: stronger attraction between particles
Solids: Very strong attraction between particles
Also known as Van Der Waal’s forces

Dispersion Forces

Forces existing between 2 polar or non-polar molecules
Formed from the electrostatic force of attraction between 2 oppositely charged temporary/induced dipoles
Electrons move randomly; thus dipoles are temporary, as their charges is based on these electrons, as their localisation is temporary
Molecules with dipoles can polarise other molecules and make them form temporary dipoles - these are called induced dipoles
This ESF of attraction between the temporary and induced dipoles are known as dispersion forces
These are temporary, as electrons move randomly, therefore the dipoles form, break apart, and reform randomly

Forces existing between an ion and a polar molecule
Formed from the electrostatic force of attraction between an ion and a dipole
Note: “Intermolecular force” is a misnomer, as this is in ionic substances, not covalent substances
Stronger than permanent dipole-dipole and dispersion forces

Forces present between 2 polar molecules
Formed from the electrostatic force of attraction between 2 net dipoles; i.e. partially positive end of one dipole, and partially negative end of another, and vice versa
Stronger than dispersion forces
- They are permanent
- There is a greater separation of charge, and therefore a greater ESF of attraction between the 2 dipoles
Stronger than intramolecular forces
Relatively weak compared to metallic, ionic, and covalent bonds

Strong dipole-dipole attraction between an electron deficient hydrogen atom attached to an electronegative atom (N, O, F), and the lone pair of electrons on a highly electronegative atom on a different molecule $H^{δ +} - F^{δ -} ∣∣∣∣∣∣∣∣∣ H^{δ +} - F^{δ -}$
- Hydrogen is electron deficient, Fluorine is electronegative
- Strong dipole-dipole forces
- Hydrogen bonding represented as |||||||
The bonds are stronger than dipole-dipole forces and dispersion forces
Strongest of all intermolecular forces
- They are permanent
- There is a greater separation of charge, and therefore a greater ESF of attraction between the 2 dipoles
Responsible for unique properties of water
- Water has an interesting boiling point; $100˚ C$ , low boiling point, but relatively high for its size
- Universal solvent; polar compound; dissolves ionic compounds easily
- Very high surface tension
- Relationship between ice and water; ice floats in water, even though it shouldn’t, as a solid
  - Ice is less dense than water
  - Hydrogen bonds become permanent when water freezes
  - Water molecules are held together in a unique 3D structure