LONDON DISPERSION FORCES | Instantaneous Dipole Induced Dipole Forces| Definition, Examples & Concept Points|

London dispersion forces or instantaneous dipole induced dipole forces originate among non polar molecules. These are the weakest forces among intermolecular forces 

How London Forces Originate?

All atoms and molecules have constant moving electrons around nucleus - in non polar molecules these electrons are evenly distributed called symmetrical distribution- when 1st non polar molecule with symmetrical distributed electrons by chance get disturb and  gain position in such a way that molecules become dipole- these will be called unsymmetrical electron distribution. Now this 1st molecule is having instantaneous dipole. This molecule will induce Dipole in nearby molecule by attracting its electrons towards its instantaneous positive pole. In this way 2nd molecule will generate induced Dipole. Hence these forces are also known as instantaneous Dipole Induced Dipole Forces.

Pictorial description of London Forces.

Strength of London forces:

Strength of London forces depends on size of molecules. Greater the size stronger will be forces. In fact molecules with bigger size can spend more time in( distorted shape) unsymmetrical distribution of electrons hence produce strong forces. 

We can relate strength of London forces with Polarizibility. Molecules with greater Polarizibility will show strong forces.

Polarizibility is the quantitative measurement of extend, a molecule be polarized. 

There are two terms to remember.

1. Polar molecules
2. Polarized molecules

Polar molecules:

Polar molecules are those molecules which have permanent poles like H2O, HCl, HF, and NH3 etc.

Polarized molecules:

Polarized molecules are those, Which get poles temporarily. Noble gases( He, Ne, Ar, Kr and Xe), homo diatomic molecules( F2,Cl2, Br2, I2,H2) CO2, CH4 .

Note:

• London forces exist in all type of molecules polar and non polar molecules.
• In polar molecules dipole-dipole forces cause to ignore these weaker forces
• While in non polar molecules only these forces exist and deal with physical properties ( thermodynamics properties) like physical states, melting and boiling points etc.