Van Der Waals forces; changing electostatic charge producing electrostatic force?

[rabbitweed]

I was reading through my materials book for Eng 101 when I came across this, on van der Waals bonds;

...[van der Waals bonds are] formed when the fluctuating electrostatic charge in adjacent atoms of different molecules produces a weak electrostatic force between the molecules

Ok, I get that Electrostatic Force can be measured by Coulombs Law;

F = k*q1*q2*/r^2

But I am struggling to understand how a "Fluctuating electostatic charge" prodces that force.

I mean, even if q1 and q2 remain constant, there's still going to be a force (mathematically at least), so why do the forces need to be fluctuating?

Also, if a van der Waals bond requires a fluctuating charge, does this mean they can only exist in molecules with covalent or metallic bonds, where the charge can change easily? AFAIK ionic charges are pretty fixed.

[rabbitweed]

hold up, I may be onto something...

if this because two neutral balanced atoms would have a charge of 0, and therefore

F = k*q1*q1/r^2

would be

F = k*0*0/r2 = 0 N

??

So it's all based on the assumption that if the charge is nonfluctuating it's likely to be 0?

Man I have a headache.

[AndroidD5]

Ok i think i get your point.....
Now...
I mean, even if q1 and q2 remain constant, there's still going to be a force (mathematically at least), so why do the forces need to be fluctuating?
An atom is made up of electrons and nucleus....consider it to be like a rubber ball that is stretchable....So between two molecules there would be 4 forces(permanent)2-repulsion and 2-attraction...and in addition there is is vanderwaal force due to unsymmetricity in the electron cloud(say the rubber portion of the rubber ball moves more towards one direction)...Now due to this unsymmetricity temporary dipoles are made....which then again induce dipoles in other molecules..remember this process only happens for a very very short intervals....BTW their effect is more pronounced in covalent bons

[Ralili]

Ok i think i get your point.....
Now...

An atom is made up of electrons and nucleus....consider it to be like a rubber ball that is stretchable....So between two molecules there would be 4 forces(permanent)2-repulsion and 2-attraction...and in addition there is is vanderwaal force due to unsymmetricity in the electron cloud(say the rubber portion of the rubber ball moves more towards one direction)...Now due to this unsymmetricity temporary dipoles are made....which then again induce dipoles in other molecules..remember this process only happens for a very very short intervals....BTW their effect is more pronounced in covalent bons

Okay, this guy explained it rather nicely. The Van der Waals forces can be caused either by polar molecules or by apolar molecules. It's in the last ones where it is caused by an unsymmetrical disposition of the electrons, forming a dipole (diferent densities of charge at the different places of the molecule) which at its time induces dipoles to the molecules nearby (this is called the London dispersion force) creating a force that maintains these molecules "tied" together.

[rabbitweed]

Ok i think i get your point.....
Now...

An atom is made up of electrons and nucleus....consider it to be like a rubber ball that is stretchable....So between two molecules there would be 4 forces(permanent)2-repulsion and 2-attraction...and in addition there is is vanderwaal force due to unsymmetricity in the electron cloud(say the rubber portion of the rubber ball moves more towards one direction)...Now due to this unsymmetricity temporary dipoles are made....which then again induce dipoles in other molecules..remember this process only happens for a very very short intervals....BTW their effect is more pronounced in covalent bons

So the fluctuating force is mainly due the electron "probability cloud", which induces dipoles?

Oh that makes sense. Thanks.

[AndroidD5]

So the fluctuating force is mainly due the electron "probability cloud", which induces dipoles?

Oh that makes sense. Thanks.
Yes...dipoles of temporary nature...
well you are welcome...(BTW why doesnt my thanked post doesnt show that i hav been thanked 1 more time??:(:):D)