Explaining Intermolecular Forces at the Quantum Level

In summary, the conversation discusses the quantum-level explanations for various mechanical properties of materials such as adhesion, cohesion, strength, stiffness, strain energy, vibration, and intermolecular forces. The topic of van der Waals forces is brought up as a potential explanation for these properties, but a complete quantum description is not feasible in a single message-board post. The original poster, a structural engineer, is specifically interested in the differences in intermolecular forces between two closely spaced solids and their individual molecules.
  • #1
mdshameer
2
0
Can someone enlighten me about the quantum level explanations for the following:
1. Adhesion between two substances
2. Cohesion within a substance
3. Strength of a substance (as in tensile strength, compressive strength and shear strength)
4. Stiffness of a substance (as in Youngs modulus, Poissons ratio and shear modulus)
5. Strain energy of materials
6. Vibration and damping of strain energy systems
7. Variation of inter molecular forces with tempearture (as in melting and boiling)
8. Variation of intermolecular forces with external pressure
9. Absence of intermolecular forces between two soilds however closely they are spaced
Hope that I have expressed my queries clearly.
Regards
 
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  • #2
"van der Waals" forces, in its original and most general form, refers to essentially all intermolecular forces (except possibly hydrogen-bonding, which at least in practice is excluded even from the most general usages these days). You might want to start http://en.wikipedia.org/wiki/Intermolecular_force" [Broken].

There certainly exists quantum-level explanations for all that, but there's no way it could be summarized in a single message-board post. You're basically asking for a complete quantum description of most chemical and material properties, which is something that encompasses several whole fields (quantum chemistry, solid-state physics). So you'd really need several whole textbooks to get the details.

I'm not sure what you mean by (9) though. There are always intermolecular forces, even at a large distance (at the very least there are always London forces).
 
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  • #3
Dear Alxm, thanks for the reply.
I am however, not asking for a complete description of most chemical and material properties. I am a structural engineer by profession and am interested only in the mechanical properties of a continuum. Besides, I am not also looking for a detailed description - sources are more than sufficient. Unfortunately, I could not dig out muc from the net, myself.
The 9 th point is the particular position that puzzles. Please forgive me if I am wrong. Suppose, I take two bars of steel and place them extremely close side-by-side. Intutively I feel that the forces between molecules within one bar and the forces between close molecules from bar-to-bar are different. If so, what is the origin of such difference?
I hope I have made the query clear
Regards
 

What are Van der Waals forces?

Van der Waals forces are weak intermolecular forces between molecules or atoms. They are caused by the temporary dipoles that occur when electrons in one molecule are unevenly distributed, creating a temporary attraction to electrons in a neighboring molecule.

What are the three types of Van der Waals forces?

The three types of Van der Waals forces are London dispersion forces, dipole-dipole interactions, and hydrogen bonding. London dispersion forces occur between all molecules and are the weakest type of Van der Waals force. Dipole-dipole interactions occur between polar molecules, and hydrogen bonding occurs between a hydrogen atom bonded to an electronegative atom and another electronegative atom.

How do Van der Waals forces affect the properties of substances?

Van der Waals forces can affect the boiling and melting points of substances, as well as their viscosity and surface tension. They can also play a role in the solubility of substances in different solvents.

Can Van der Waals forces be stronger than covalent or ionic bonds?

No, Van der Waals forces are much weaker than covalent or ionic bonds. They are only significant in molecules and substances that do not have strong covalent or ionic bonds between their atoms.

How is the strength of Van der Waals forces affected by molecular size?

The strength of Van der Waals forces increases with increasing molecular size. This is because larger molecules have more electrons, creating stronger temporary dipoles and thus stronger Van der Waals forces.

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