London Dispersion Forces
Chemistry: IB Diploma Higher Level, topic 2.8.2
Induced Dipole-Dipole Interactions
Induced dipole-dipole interactions are also known as London dispersion forces. They are formed from temporary dipoles.
Temporary dipoles
- The electron clouds around molecules are constantly in motion.
- One moment, all the electron density can be on one side of the molecule. The next, it can be somewhere totally different.
- This is called a temporary dipole - there are partial charges, but they change very rapidly.
Induced dipoles
- If one molecule has a temporary dipole, its partial charges will exert a force on nearby molecules.
- The partial charge of one molecule can push away the electrons in another, or attract them towards it.
- This means that temporary dipoles will induce dipoles in nearby molecules.
- Once a dipole has been induced, it will be attracted to the initial dipole.
- This is called an induced dipole interaction, or a van der Waals force.
Strength
- London dispersion forces are not all the same strength.
- The strength depends on the number of electrons in a molecule.
- Molecules with lots of electrons will have much stronger London dispersion forces.
- This is because they will have larger fluctuations in electron density.
- This leads to larger temporary dipoles and stronger dipole-dipole interactions.
- The presence of electron-dense π-bonds also increases the strength of London-dispersion forces.
Test yourself on London Dispersion Forces
What are 'London dispersion forces' also known as?
What is the strength of London dispersion forces interactions dependant on?