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Can someone explain Van Der Waal/London forces to me as simple as possible? Along with explaining Permanent dipole-dipole forces?


nae
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Posted (edited)
58 minutes ago, nae said:

I have a hard time understanding the intermolecular forces except hydrogen bonding but the rest is just like...???

Permanant dipoles are easy to explain. You have a molecule with partial +ve charge in one place and a partial -ve charge somewhere else. The partial +ve charge will attract a partial -ve charge in a neighbouring molecule and vice versa. So it's just like the attraction between oppositely charged ions but involving only partial charges. 

London forces, also known as dispersion forces, arise due to "flickering, fleeting dipoles" due to motion of the electrons in an atom or molecule, which induce dipoles in the neighbouring ones. The strength of dispersion forces is greater between atoms (or molecules involving them) that have greater polarisability, which tends to mean larger atoms with a more diffuse outermost shell of electrons. As I recall, the random fluctuations in electron density that give rise to this arise from the same quantum mechanical principle responsible for vacuum fluctuations - basically another manifestation of the uncertainty principle.  

The name Van der Waals forces is given to all intermolecular attractions that don't involve a chemical bond. So the term includes both London (dispersion) forces and the attraction between permanent dipoles.  (But it would not include hydrogen bonds, as these have some  directional bonding character and are thus not entirely electrostatic dipole attractions.)

Edited by exchemist
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2 hours ago, nae said:

Can someone explain Van Der Waal/London forces to me as simple as possible? Along with explaining Permanent dipole-dipole forces?

I have a hard time understanding the intermolecular forces except hydrogen bonding but the rest is just like...???

1 hour ago, exchemist said:

[...]The name Van der Waals forces is given to all intermolecular attractions that don't involve a chemical bond. So the term includes both London (dispersion) forces and the attraction between permanent dipoles.  (But it would not include hydrogen bonds, as these have some  directional bonding character and are thus not entirely electrostatic dipole attractions.)

13 minutes ago, MigL said:

Simplest explanation possble ...
Residual Electromagnetic forces.

J. Diderik van der Waals's "inter-molecular" forces explanation is distance-dependent, and are "comparatively weak, and vanish at longer distances", theoretically. 

Quote

The van der Waals forces are usually described as a combination of the London dispersion forces between "instantaneously induced dipoles", Debye forces between permanent dipoles and induced dipoles, and the Keesom force between permanent molecular dipoles whose rotational orientations are dynamically averaged over time.

To complicate the matter further, I ask: are we dealing with nuclear droplets, or have we, "flown to the Moon" model, of nuclear structure? I for one do not understand hydrogen bonding, either, and need to review the basics.

+1 for exchemist's explanation, and I agree with MigL there is not much to add to address the question properly (caveat: Again, I need to review even these basics). In my opinion the better formulation of Occam's razor states that the explanation needs to be as complicated as needs be dictated by the nature of the question posed...

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Posted (edited)
22 minutes ago, NTuft said:

J. Diderik van der Waals's "inter-molecular" forces explanation is distance-dependent, and are "comparatively weak, and vanish at longer distances", theoretically. 

To complicate the matter further, I ask: are we dealing with nuclear droplets, or have we, "flown to the Moon" model, of nuclear structure? I for one do not understand hydrogen bonding, either, and need to review the basics.

+1 for exchemist's explanation, and I agree with MigL there is not much to add to address the question properly (caveat: Again, I need to review even these basics). In my opinion the better formulation of Occam's razor states that the explanation needs to be as complicated as needs be dictated by the nature of the question posed...

This is all about the behaviour of electrons in atoms and molecules, not the nuclei. 

Hydrogen bonding remains I think something of an enigma. At one time there was a view that it was just a special case of an attraction between permanent dipoles, but in fact it has directional character, which seems to involve the electrons of the "lone pairs" of electrons on the electronegative atom. So there seems to be an element of electron pair sharing, as in a covalent bond.   

Edited by exchemist
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Posted (edited)
16 minutes ago, exchemist said:

This is all about the behaviour of electrons in atoms and molecules, not the nuclei. 

Au contraire, mon ami -- the two are inexorably linked I think we would agree, and the Moon model posits tandems of nucleons at discrete points on a definite structure, as I understand it. An analogy would be crystal structure. I do not understand it, and need to read and study more.

16 minutes ago, exchemist said:

Hydrogen bonding remains I think something of an enigma. At one time there was a view that it was just a special case of an attraction between permanent dipoles, but in fact it has directional character, which seems to involve the electrons of the "lone pairs" of electrons on the electronegative [ed.:electronegativity deserves emphasis to the question at hand] atom. So there seems to be an element of electron pair sharing, as in a covalent bond.

Hydrogen does seem to be acting funny, but it doesn't have much of a shell now, does it?.. It is like an iota. The table is set with many and various names from the lexicon but some see it as iterations on an iota, or an iota that is paired and by any measure significant. I have not processed what you've actually written and may be responding mechanically. -1 me to 0.

Edited by NTuft
emphasis; correction on definite
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1 minute ago, NTuft said:

Au contraire, mon ami -- the two are inexorably linked I think we would agree, and the Moon model posits tandems of nucleons at discrete points on a define structure, as I understand it. An analogy would be crystal structure. I do not understand it, and need to read and study more.

 

The role played by atomic nuclei in chemical bonding and intermolecular attractions is to provide the potential wells that confine the electrons in their orbitals. The variety of forms of attraction between atoms arises from the ways in which the electrons in adjoining atoms behave.   

The "Moon model", put forward in the 1980s by Robert J Moon and apparently not taken seriously today, concerns nuclear structure. This has no impact at all on chemical bonding.   

 

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On 5/16/2022 at 6:42 PM, nae said:

I have a hard time understanding the intermolecular forces except hydrogen bonding but the rest is just like...???

 

Is this for examination purposes ?

I ask because UK A level exams are coming up and I was preparing some more detail for you but there is a small but important point I would like to clear up.

Van der Waals proposed his froces in 1873, before experimental confirmation of either atoms or molecules.

London proposed his quantum idea in 1930 to explain the work on inttermolecular forces that had taken plece in the intervening half century.

Since then modern Chemistry has gained a vast amount of new data, both the VDW and London explanations have been refined and redefined several times.

So it is important to use he definitions and explanations appropriate to your syllabus if this is exam material.

The UK A level currently defines VDW as a general label for intermolecular forces and London forces as a special case.

 

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On 5/16/2022 at 2:05 PM, exchemist said:

The role played by atomic nuclei in chemical bonding and intermolecular attractions is to provide the potential wells that confine the electrons in their orbitals. The variety of forms of attraction between atoms arises from the ways in which the electrons in adjoining atoms behave.   

The "Moon model", put forward in the 1980s by Robert J Moon and apparently not taken seriously today, concerns nuclear structure. This has no impact at all on chemical bonding.   

 

My point of contention would be that an extended nuclear structure is what makes possible the exposure of a positive dipole, and that this does play a role in nucleophilic-electrophilic bonding, for example. That said, I'm well far off from what should be addressed here so my apologies for that.

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1 hour ago, NTuft said:

My point of contention would be that an extended nuclear structure is what makes possible the exposure of a positive dipole, and that this does play a role in nucleophilic-electrophilic bonding, for example. That said, I'm well far off from what should be addressed here so my apologies for that.

What is a positive dipole ?

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10 hours ago, NTuft said:

My point of contention would be that an extended nuclear structure is what makes possible the exposure of a positive dipole, and that this does play a role in nucleophilic-electrophilic bonding, for example. That said, I'm well far off from what should be addressed here so my apologies for that.

Not at all. The dipole is due to a distribution of electron density that is offset, to some extent, from the +ve charges of the atomic nuclei. The internal structure of the nuclei has no bearing at all on this. The dimensions of atomic nuclei are far too small compared to the dimensions of the cloud of electrons. A molecule like H-Cl has a dipole because the electrons in the bond between the atoms are biased more towards the Cl atom than the H atom, giving the H atom a partial +ve charge and the Cl atom a partial -ve charge. That effect arises due to the way the electrons occupy successive quantum mechanical states, starting with those of lowest energy. Elements on the right of the p-block of the Periodic Table have valence orbitals that experience a strong nuclear charge in relation to their average distance from the nucleus, whereas those in the succeeding s-block are in the next quantum shell out, so they are not attracted as strongly by the nucleus. It is all to do with the quantum states available to the electrons.    

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On 5/19/2022 at 4:18 PM, studiot said:

What is a positive dipole ?

I say the positively charged proton exposed due to electrons orbiting a ways from that nucleon.

On 5/20/2022 at 1:22 AM, exchemist said:

Not at all. The dipole is due to a distribution of electron density that is offset, to some extent, from the +ve charges of the atomic nuclei. The internal structure of the nuclei has no bearing at all on this. The dimensions of atomic nuclei are far too small compared to the dimensions of the cloud of electrons.

I do not think this can be taken for granted, and the prior statement, "the internal structure of the nuclei has no bearing at all on this", actually follows from the second statement.

On 5/20/2022 at 1:22 AM, exchemist said:

A molecule like H-Cl has a dipole because the electrons in the bond between the atoms are biased more towards the Cl atom than the H atom, giving the H atom a partial +ve charge and the Cl atom a partial -ve charge. That effect arises due to the way the electrons occupy successive quantum mechanical states, starting with those of lowest energy. Elements on the right of the p-block of the Periodic Table have valence orbitals that experience a strong nuclear charge in relation to their average distance from the nucleus, whereas those in the succeeding s-block are in the next quantum shell out, so they are not attracted as strongly by the nucleus. It is all to do with the quantum states available to the electrons.

Thank you. I need to review even spectral line data. And then to the Linear-Combinations-of-Atomic-Orbitals (LCAO). Even the basics of QM and the four numbers, which are describing the electrons... Thanks again.

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21 hours ago, NTuft said:

Thank you. I need to review even spectral line data. And then to the Linear-Combinations-of-Atomic-Orbitals (LCAO). Even the basics of QM and the four numbers, which are describing the electrons... Thanks again.

I think you need to look elsewhere.

Your nearly had the right idea when you replied to my question.

21 hours ago, NTuft said:
On 5/20/2022 at 12:18 AM, studiot said:

What is a positive dipole ?

I say the positively charged proton exposed due to electrons orbiting a ways from that nucleon.

The point for this thread is that standard valency bonds are permanent, as are permanent dipole interactions which do not form valency bonds.
Both of these types can hold solids together. The non valency ones are a type of VDW force.
But yes, a further interaction can be due to a temporary dipole. These are the London forces.
Both temporary and permanent dipoles have two possible sources of derivation.

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