Van der Waals attractive force: Why don't the imbalances average out?

In summary, the Van der Waals force is a type of intermolecular force that results from a temporary shift in electron density near the nucleus of an atom. This can cause an induced charge that can either attract or repel nearby atoms. The strength of this force is dependent on the distance between atoms, with the optimal distance being between 0.4 and 0.6 nanometers. Induction plays a key role in this force, as one atom can induce a polarisation in another atom, resulting in attraction between them.
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nomadreid
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TL;DR Summary
When a van der Waals force between two objects is attractive (before getting too close), it is due to the variable distribution of electrical charge between the molecules. But why wouldn't this variation average out (+ + /+ -/- +/- -) to give zero?
The summary refers, for example, to the Wikipedia explanation
"The force results from a transient shift in electron density. Specifically, the electron density may temporarily shift more greatly to one side of the nucleus. This generates a transient charge to which a nearby atom can be either attracted or repelled."
https://en.wikipedia.org/wiki/Van_der_Waals_force

(for a distance between atoms of between 0.4 to 0.6 nanometers, as explained in that article)

Also, when I speak of the average (+ + /+ -/- +/- -) , this would include the sum (integral) of all the different strengths according to the angles, distances, etc. But the idea is still the same.
 
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  • #2
Consider two close spheres, (±) (±). One becomes momentarily polarised, +()– (±). The other sphere then takes an induced polarisation in the same direction, +()– +()–. The closest points on the spheres have opposite charge so attraction dominates.
 
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  • #3
Thank you, Baluncore. I had not considered induction. Question answered! :smile:
 
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1. What is the Van der Waals attractive force?

The Van der Waals attractive force is a weak intermolecular force that exists between molecules. It is caused by temporary imbalances in the distribution of electrons, creating a slight positive and negative charge on different parts of the molecule. This results in a weak attraction between molecules.

2. How does the Van der Waals force differ from other types of intermolecular forces?

The Van der Waals force is different from other intermolecular forces, such as hydrogen bonding or dipole-dipole interactions, because it is caused by temporary imbalances in electron distribution rather than permanent dipoles or hydrogen bonds.

3. Why don't the imbalances in electron distribution average out?

The imbalances in electron distribution do not average out because they are constantly changing. As molecules move and vibrate, the distribution of electrons also changes, resulting in a constantly fluctuating attractive force between molecules.

4. How does the Van der Waals force contribute to the properties of materials?

The Van der Waals force plays a significant role in determining the physical properties of materials. It can contribute to properties such as melting and boiling points, viscosity, and surface tension. It also affects the interactions between molecules, which can impact the strength and stability of materials.

5. Can the Van der Waals force be manipulated or controlled?

The Van der Waals force is a natural phenomenon that cannot be directly manipulated or controlled. However, its effects can be altered by changing the physical and chemical properties of molecules. For example, increasing the size or polarizability of molecules can increase the strength of the Van der Waals force between them.

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