[Kayin]

I've been studying through these notes about intermolecular theory about
dipole-dipole forces
ion-dipole forces
Dispersion forces...

There's some things I don't understand, do all pairs of atoms have dispersion forces? And the notes also say "If an ion or a polar molecule is close enough ti can induce a temporary dipole in an atom". Well, how can you tell if an ion or polar moleculre is "close enough"?  

[Mitch]

If its within vanderwalls distance.