dipole dipole and dispersion

[claud08]

can some please explain the difference and give some examples?   

[TrueTears]

A dipole molecule is where one end has a partial positive charge and the other end has a partial negative charge.

EG, HCl

H ---- Cl The Cl has the partial negative charge because it is more electronegative

Now dipole to dipole attractions are an intermolecular type of bonding. Take again the example of HCl, these HCl molecules line up so that the partial negative charges are with the partial positive charges and vice versa

          H ---- Cl
H ---- Cl
          H ---- Cl

[claud08]

yer i know what dipole is but i don't understand what dispersion is let alone the difference

[TrueTears]

Now dispersion is when the molecule is symmetrical and does not have 2 'poles'

Take for example.

Cl ---- Cl

The electron cloud is evenly distributed, there is no dipole, hence this molecule is not a dipole molecule.

Now dispersion forces occur when at some instant in time the electron cloud will be more to the Cl on the right giving it a slight negative charge, so it becomes polarized instantaneously. But another moment in time the electron cloud will be more to the Cl on the left give it a slight negative charge.

Dispersion forces become most important when it is the only force between particles.

[Collin Li]

Dispersion forces arise from instantaneous dipoles (due to the random nature of the electron movement). You can intuitively think of it as "less reliable" as it's based on chance, and hence the bonds are weaker than dipole-dipole interactions, which involve permanent dipoles (not dipoles that only last for an instant)