[PicturesOfLilly]

Shouldn't van der Waal forces of repulsion occur equally has often as van der Wall forces of attraction? I understand why there are the forces of attraction, but if you think about it there should also beforces of repulsion. Why is it that non polar compounds have an over all force of attraction within their molecules?

[Vidya]

van der Waal forces of repulsive forces ...find out when they become dominating ? or when their nuclei or electrons start repelling?

Find out what happens to charge distribution  in non polar compounds which can cause attraction?

[Irlanur]

There are... it depends on the distance. See for example https://de.wikipedia.org/wiki/Lennard-Jones-Potential

[pyrophoric]

In biochemistry, hydrophobic interactions are better explained by entropy and the reordering of water around the nonpolar molecule (solvation cages) that provides the driving force aggregating non polar molecules together. 

[Arkcon]

In biochemistry, hydrophobic interactions are better explained by entropy and the reordering of water around the nonpolar molecule (solvation cages) that provides the driving force aggregating non polar molecules together.

This seems off topic to me.  Do you want to relate these comments specifically to the O.P. in some way?

[pyrophoric]

off topic?  OP asked the question "why is it that non polar compounds have an overall force of attraction within their molecules?"  That was the answer.  There is no net attractive force between nonpolar molecules, it is the minimization of interactions between nonpolar molecules and water that increases entropy and drives nonpolar molecules together.  It's a more stable system that way.  Not sure why you think this is "off-topic".  Maybe you didn't understand the question, not sure. 

[Irlanur]

There is no net attractive force between nonpolar molecules

Of course there are, they are just much weaker.

[pyrophoric]

There is no net attractive force between nonpolar molecules

Of course there are, they are just much weaker.

No, it's not a net attractive force if the interaction varies because it is caused by induced dipoles with polarizable electron clouds which are just as likely to attract or repel depending on the environment.  That's not a net attraction at all, it's actually a net repulsion from the dipoles of water caused by H-bonding if anything. 

[mjc123]

Who said anything about water? You mentioned "in biochemistry", but OP didn't. Why do nonpolar compounds liquefy?

[pyrophoric]

Who said anything about water? You mentioned "in biochemistry", but OP didn't. Why do nonpolar compounds liquefy?

I said "in biochemistry" as a qualifier.  Nonpolar compounds liquefy because they have a melting point, a temperature where the physical state changes from solid to liquid at a specific atmospheric pressure.  The atmosphere contains water on Earth, I assume OP meant a nonpolar compound on Earth. . 

[Babcock_Hall]

@OP,
If one tried to force any two molecules or any two groups on a single molecule too close together, the repulsive forces are stronger.  The gauche conformation of butane has higher energy than the anti conformation because of repulsion between the two methyl groups.

[Irlanur]

No, it's not a net attractive force if the interaction varies because it is caused by induced dipoles with polarizable electron clouds which are just as likely to attract or repel depending on the environment.  That's not a net attraction at all, it's actually a net repulsion from the dipoles of water caused by H-bonding if anything. 

Ok, let's ignore gas phase studies, theoretical chemistry and MD simulations then... of course it depends on the distance between the molecules, but that's always the case, also with any other type of "chemical" interaction. Just look at the Lennard-Jones potential.

Nonpolar compounds liquefy because they have a melting point, a temperature where the physical state changes from solid to liquid at a specific atmospheric pressure.

That's not an explanation at all, you're just rephrasing the question. So why can it be liquid/solid at all? Do you want to imply that the water is actually necessary for this?

[Babcock_Hall]

Shouldn't van der Waal forces of repulsion occur equally has often as van der Wall forces of attraction? I understand why there are the forces of attraction, but if you think about it there should also beforces of repulsion. Why is it that non polar compounds have an over all force of attraction within their molecules?

Consider hexane, which is a liquid at room temperature.  There must be forces of attraction, or it would be a gas.  On the other hand, it has a finite volume and shows little change in volume upon compression.  What does this information suggest to you?

[pyrophoric]

My point is that nothing exists in a vacuum, there is always an entropy barrier to account for, and with nonpolar molecules it doesn't have to be very large to have more an effect than the 0.05 - 25 kJ bonds of van der waals forces.  But at least we got some good discussion out of it, some decent replies and what not. 

[pyrophoric]

Nonpolar compounds liquefy because they have a melting point, a temperature where the physical state changes from solid to liquid at a specific atmospheric pressure.

That's not an explanation at all, you're just rephrasing the question. So why can it be liquid/solid at all? Do you want to imply that the water is actually necessary for this?
[/quote]

Changing the physical state changes the entropy and the corresponding potential energy in the environment, you can't increase entropy without it expecting to have an effect on the surroundings, and those surroundings are going to push back, in whatever physical state they are in.  The actual point of all this is that the so called affinity between nonpolar molecules is SO WEAK that it is extremely susceptible to external forces.  And in biochemistry, those forces are water which are so strong that they redefine the entire structure of nonpolar molecules because that so called affinity pales in comparison.