What is "touch" in molecular terms? @_@

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starfire09
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What is "touch" in molecular terms? @_@

Postby starfire09 » Sat Sep 05, 2009 4:24 am UTC

Hey everyone. So for my first post on the XKCD forums I decided to ask a question that has bugged me since my freshman year of high school. I'm a chemistry major with a little bit of history in physics (I frequent science forums and get a lot of my physics knowledge from that [I'm really into theoretical astronomy and stuff. Black holes, etc.]). As I said in the topic title, my question is what exactly is the sensation of touch? I know that we have nerve ending, etc. on our skin that allows us to feel vibrations, but obviously nerve endings and our skin is composed of atoms, which is where my uncertainty (lulz, atoms and uncertainty) arises.

In my mind, I imagine somebody poking me in the arm with his finger (In incredible slow-motion if it makes a difference). Now for any actual contact to take place, the outer-most of the atoms of the skin on my arm would have to collide with the outer-most atoms of the skin of his fingertip. However, I know from nuclear chemistry that we would probably blow up if that happened. The only thing I can think of is that the sensation we attribute to "touch" is a repulsion between positive and negative components in both parties, but that seems a little strange. Any physics people around to set me straight?

Also, a related question. It's been a while since my last physics course, but IIRC repulsive forces increase as distance decreases, so would the "flexing" of our skin when it is poked related to positive nuclei coming closer together and trying to "get away"? I'm sure the answer to my first question will answer this one, but it's another thing that's bugged me since I can't figure it out.

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Re: What is "touch" in molecular terms? @_@

Postby Squid Tamer » Sat Sep 05, 2009 4:37 am UTC

I have definitely heard that atoms never actually touch, but are repulsed from each other once they get at a very close. I don't know the details, though.

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Re: What is "touch" in molecular terms? @_@

Postby iblis.raeb » Sat Sep 05, 2009 4:38 am UTC

I don't know the technical definition of touch; however, in my layman mind, touch, just like a mild electric shock, is probably an exchange of electrons.
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Re: What is "touch" in molecular terms? @_@

Postby starfire09 » Sat Sep 05, 2009 4:42 am UTC

See but an exchange of electrons implies a reaction, and with the billions of things we touch everyday of our lives, you'd think that there would be some visible changes in our skin from billions of chemical reactions (Plus the carbon [Which is what most of the proteins in our bodies are made of. This is purely skeptical and based off Organic Chemistry as I'm not planning on taking any biochem] that our skin would have "able" to bond already has a full valence shell, so it wouldn't be able to accept more electrons).

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Re: What is "touch" in molecular terms? @_@

Postby iblis.raeb » Sat Sep 05, 2009 4:44 am UTC

starfire09 wrote:See but an exchange of electrons implies a reaction, and with the billions of things we touch everyday of our lives, you'd think that there would be some visible changes in our skin from billions of chemical reactions (Plus the carbon [Which is what most of the proteins in our bodies are made of. This is purely skeptical and based off Organic Chemistry as I'm not planning on taking any biochem] that our skin would have "able" to bond already has a full valence shell, so it wouldn't be able to accept more electrons).

I am fairly certain that we shed skin regularly.
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Re: What is "touch" in molecular terms? @_@

Postby psychosomaticism » Sat Sep 05, 2009 4:46 am UTC

Yep. From my limited knowledge of physics, atoms will repel each other, and probably cause some sort of electric difference in doing so, or at least a potential of some sort (probably not voltage, unless static counts). Whatever the result, our mechanoreceptors in our skin/organ will pick up on these, whether they be vibration, deep pressure, etc (see wikipedia). In conclusion, no, we never actually touch anything because atoms don't touch, but that doesn't mean touch isn't happening; it says a lot about perception and reality.

As for atomic repulsion acting in skin stretch, I really doubt that's the main factor. Yes, that will happen on a molecular level, but because we're solid, the atoms won't move far. It's more likely that it's simple classical mechanics of force and elasticity of skin and muscle.

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Re: What is "touch" in molecular terms? @_@

Postby starfire09 » Sat Sep 05, 2009 4:51 am UTC

psychosomaticism wrote:Yep. From my limited knowledge of physics, atoms will repel each other, and probably cause some sort of electric difference in doing so, or at least a potential of some sort (probably not voltage, unless static counts). Whatever the result, our mechanoreceptors in our skin/organ will pick up on these, whether they be vibration, deep pressure, etc (see wikipedia). In conclusion, no, we never actually touch anything because atoms don't touch, but that doesn't mean touch isn't happening; it says a lot about perception and reality.

As for atomic repulsion acting in skin stretch, I really doubt that's the main factor. Yes, that will happen on a molecular level, but because we're solid, the atoms won't move far. It's more likely that it's simple classical mechanics of force and elasticity of skin and muscle.



Yeah I guess I figured that the majority of touch is our brain perceiving what's happening and stuff. Damn, I just wish there was a straight answer, you know what I mean? Stupid brain, stop raising questions >=O

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Re: What is "touch" in molecular terms? @_@

Postby psychosomaticism » Sat Sep 05, 2009 4:53 am UTC

I suppose the initial question is a bit flawed, though. Touch doesn't really exist in molecular terms, and we don't perceive the world in those terms either. Sort of like how quantum physics and classical mechanics don't really mix well sometimes. I think it's an okay explanation, and part way true, that when we get close enough to another atom we get an impulse from our receptors and get action potential up our axons to the brain, etc.

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Re: What is "touch" in molecular terms? @_@

Postby starfire09 » Sat Sep 05, 2009 4:58 am UTC

Yeah I see what you mean. I mean it's "obvious" that touch is possible, since we use our tactile senses every day, but in my mind I try to zoom in on the contact point and see what's going on in the smallest terms possible, then things get difficult to ponder.

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Re: What is "touch" in molecular terms? @_@

Postby Gelsamel » Sat Sep 05, 2009 5:09 am UTC

"Touch" -is- happening... it's just not what you think it is. The sensation of feeling is just caused by interactions of magnetic fields intra-nucleus. It's the interaction between the positive charges of different nuclei because they are concentrated magnetic charges whereas the electrons are distributed in a ridiculously wide area.

Defining "Touch" as direct atomic contact of solid material (nucleus) is a ridiculously naive definition and not very helpful... since this definition is essentially "nuclear fusion" and doesn't apply to any other situation (well, any other that I can think of).
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Re: What is "touch" in molecular terms? @_@

Postby Charlie! » Sat Sep 05, 2009 5:47 am UTC

Okay, here's my best shot, despite the mixing of metaphors:

When they're far away, atoms are neutral. You can think of this as because the nucleus is shielded by a coat of electrons. Like a marble hidden inside a balloon.

When you try to squoosh them against each other, though, the electron clouds blow each other away, removing the shielding of the nuclei and letting their repulsive force get a solid repulsive shot at the other.

Slightly more in-depth:
It's because the electrons are out behind the nuclei, and the inverse square law says that the far-away things matter even less than you'd expect. So if the distance between the nuclei is D and the distance between the middle of the electrons is D+2*f, then the force between the nuclei is like (i.e. proportional to) D^2, the force between the clouds is like (D+2f)^2, and the attractive force between clouds and nuclei is like 2*(D+f)^2. So the total repulsive force is proportional to D^2 + (D+2f)^2 - 2*(D+f)^2 = 3*f^2. It's all about the distance between the nuclei and their electron clouds.
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Re: What is "touch" in molecular terms? @_@

Postby dmca » Sat Sep 05, 2009 7:11 pm UTC

I'm hardly an expert on physical chemistry but none of the answers here seem particularly satisfactory so I'll give this a go.

If you simplify the question to "why should two particles repel one another as they approach" then the interaction may be approximated by the Lennard-Jones potential:
[math]V(r) = A/{r^{12}} - B/r^6[/math]
r is the distance between the two particles, A gives the short range repulsive interaction and B the longer range attraction (due to van der Waals interactions/London dispersion).

The repulsive force is, I believe primarily, due to the electrostatic repulsion of the negatively charged electrons and that the force between positive nuclei is unlikely to be significant with non-bonded atoms. I don't think the Pauli principle is particularly important except for identical particles but my quantum mechanics is rather rusty.

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Re: What is "touch" in molecular terms? @_@

Postby Tass » Mon Sep 07, 2009 7:13 am UTC

The pauli principle is indeed important. It forces the electrons into very antibonding orbitals as the nuclei get close.

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Re: What is "touch" in molecular terms? @_@

Postby dmca » Mon Sep 07, 2009 10:08 am UTC

Tass wrote:The pauli principle is indeed important. It forces the electrons into very antibonding orbitals as the nuclei get close.


Err, I'm talking about two non bonded (and non identical) particles. What you say is definitely true in say an heterodiatomic system but is really it relevant here when there are no formal bonding or antibonding orbitals that encompass the entire system?

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Re: What is "touch" in molecular terms? @_@

Postby BeerBottle » Mon Sep 07, 2009 12:38 pm UTC

I agree with dmca that it is the electrons that are mostly responsible for atoms repelling each other, at least under normal earthly pressures (i.e. maybe not inside Jupiter or something). Charlie!, atoms are not nearly so polarizable as you are suggesting! - imagine the incredibly high energy of the arrangement you describe, with the electrons all pushed to one side and the nucleus exposed. Even in HF there is some electron density around H.

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Re: What is "touch" in molecular terms? @_@

Postby Tass » Mon Sep 07, 2009 3:46 pm UTC

dmca wrote:
Tass wrote:The pauli principle is indeed important. It forces the electrons into very antibonding orbitals as the nuclei get close.


Err, I'm talking about two non bonded (and non identical) particles. What you say is definitely true in say an heterodiatomic system but is really it relevant here when there are no formal bonding or antibonding orbitals that encompass the entire system?


It matters exactly there.

Take two hydrogen atoms. They are bonded. Once their s orbitals begin to overlap both electrons will stay in the one low energy bonding orbital. Therefore they can move very close together. Until repulsion of the nuclei begins to matter and keep a stable distance.

Take two helium atoms. They are nonbonded. Initially Wan-der-Waals forces will give a slight attraction, but once their s orbitals start to overlap significantly there is a powerful repulsion. The Pauli principle forces two of the four electrons to occupy a very nonfavorable anti-bonding orbital, which quickly gets way more unfavorable than the bonding one is favorable. They are almost like hard spheres. The repulsion between nuclei is a 1/r2 effect. It couldn't account for the observed 1/r12 repulsion.

Take two hydrogen molecules. They behave much like the helium atoms. The atoms like each other two and two, but press the molecules to close together and the two bond orbitals will combine like the helium 1s' did.

Same happens when any other electron pair gets close to another (that is, when any matter gets close). Unless there is an empty low energy orbital to combine with, then it is going to repel, because of the Pauli principle.

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Re: What is "touch" in molecular terms? @_@

Postby Charlie! » Mon Sep 07, 2009 11:56 pm UTC

Tass wrote:
dmca wrote:
Tass wrote:The pauli principle is indeed important. It forces the electrons into very antibonding orbitals as the nuclei get close.


Err, I'm talking about two non bonded (and non identical) particles. What you say is definitely true in say an heterodiatomic system but is really it relevant here when there are no formal bonding or antibonding orbitals that encompass the entire system?


It matters exactly there.

Take two hydrogen atoms. They are bonded. Once their s orbitals begin to overlap both electrons will stay in the one low energy bonding orbital. Therefore they can move very close together. Until repulsion of the nuclei begins to matter and keep a stable distance.

Take two helium atoms. They are nonbonded. Initially Wan-der-Waals forces will give a slight attraction, but once their s orbitals start to overlap significantly there is a powerful repulsion. The Pauli principle forces two of the four electrons to occupy a very nonfavorable anti-bonding orbital, which quickly gets way more unfavorable than the bonding one is favorable. They are almost like hard spheres. The repulsion between nuclei is a 1/r2 effect. It couldn't account for the observed 1/r12 repulsion.

Take two hydrogen molecules. They behave much like the helium atoms. The atoms like each other two and two, but press the molecules to close together and the two bond orbitals will combine like the helium 1s' did.

Same happens when any other electron pair gets close to another (that is, when any matter gets close). Unless there is an empty low energy orbital to combine with, then it is going to repel, because of the Pauli principle.

Slight correction: since the atoms are really dipoles when close, I think it's 1/r3 repulsion. EDIT: oh, wait, you mean REALLY close.

But aside from that, it's a very interesting point. You can definitely think of it in terms of orbitals, if you assume that the two things touching have filled up all their orbitals, trying to force them into space roughly 1 bond length apart leads to the electrons going higher in energy basically by the rules of bonding. This would kick in a bit before the dipole-dipole repulsion that happens at less than 1 bond length.
Last edited by Charlie! on Tue Sep 08, 2009 3:12 pm UTC, edited 1 time in total.
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Re: What is "touch" in molecular terms? @_@

Postby dmca » Tue Sep 08, 2009 3:00 pm UTC

Take two helium atoms. They are nonbonded. Initially Wan-der-Waals forces will give a slight attraction, but once their s orbitals start to overlap significantly there is a powerful repulsion. The Pauli principle forces two of the four electrons to occupy a very nonfavorable anti-bonding orbital, which quickly gets way more unfavorable than the bonding one is favorable. They are almost like hard spheres. The repulsion between nuclei is a 1/r2 effect. It couldn't account for the observed 1/r12 repulsion.

Take two hydrogen molecules. They behave much like the helium atoms. The atoms like each other two and two, but press the molecules to close together and the two bond orbitals will combine like the helium 1s' did.



I don't particularly like your description of this in terms bonding and anti-bonding orbitals when dealing with the interactions of two closed shell atoms.

But what you're saying is more or less true when molecules approach one another very closely i.e. near to or within their Van der Waals radii. At very short range, Pauli repulsion is going to be the dominant repulsive force but at longer ranges, electrostatic repulsion between the electrons will be more significant. I was suggesting that under normal conditions the molecules would not approach so closely. However, I'm now not entirely sure this is correct, since it's difficult to see how electrostatic repulsion would ever overcome attractive London dispersion interactions. So I was probably wrong on that point.

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Re: What is "touch" in molecular terms? @_@

Postby Tass » Tue Sep 08, 2009 6:26 pm UTC

dmca wrote:I don't particularly like your description of this in terms bonding and anti-bonding orbitals when dealing with the interactions of two closed shell atoms.

But what you're saying is more or less true when molecules approach one another very closely i.e. near to or within their Van der Waals radii. At very short range, Pauli repulsion is going to be the dominant repulsive force but at longer ranges, electrostatic repulsion between the electrons will be more significant. I was suggesting that under normal conditions the molecules would not approach so closely. However, I'm now not entirely sure this is correct, since it's difficult to see how electrostatic repulsion would ever overcome attractive London dispersion interactions. So I was probably wrong on that point.


There are no electrostatic repulsion between neutral atoms. Rather there is, as you say, a slight attraction because of the dispersion. The repulsion is because of increase in the energy level once the orbitals overlap.

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Re: What is "touch" in molecular terms? @_@

Postby dmca » Wed Sep 09, 2009 12:38 pm UTC


There are no electrostatic repulsion between neutral atoms. Rather there is, as you say, a slight attraction because of the dispersion. The repulsion is because of increase in the energy level once the orbitals overlap.


Well there clearly is a repulsion between the electrons in the system, but the overall interaction (at a range where there is no overlap of the wavefunctions) is, like you say, always attractive. I have already conceded my mistake there

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Re: What is "touch" in molecular terms? @_@

Postby Soralin » Wed Sep 09, 2009 10:40 pm UTC

A little off topic, but something I've wondered, could you take a normal container, and put some anti-protons in it? Would it be possible for it's interactions with the electron cloud to keep it away from the nuclei of other atoms(assuming that it's moving slowly enough), or would it just have an increased attraction to the nucleus as it passes through?

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Re: What is "touch" in molecular terms? @_@

Postby Tass » Thu Sep 10, 2009 6:42 am UTC

Soralin wrote:A little off topic, but something I've wondered, could you take a normal container, and put some anti-protons in it? Would it be possible for it's interactions with the electron cloud to keep it away from the nuclei of other atoms(assuming that it's moving slowly enough), or would it just have an increased attraction to the nucleus as it passes through?


No, it would quickly annihilate. The electron cloud won't keep it out.

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Re: What is "touch" in molecular terms? @_@

Postby eSOANEM » Sat Sep 19, 2009 9:29 pm UTC

We've got the solution to what touch is in molecular terms but does touch exist in the world smaller than this or does it become an irrelevant concept because of the Heisenberg uncertainty Principle.
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Re: What is "touch" in molecular terms? @_@

Postby Tass » Sun Sep 20, 2009 10:58 am UTC

eSOANEM wrote:We've got the solution to what touch is in molecular terms but does touch exist in the world smaller than this or does it become an irrelevant concept because of the Heisenberg uncertainty Principle.


You can say it becomes irrelevant, yes.

You can't put definite limits on where a picoscopic object begins or ends, so the question "are they touching?" becomes meaningless.

"Touch" is simply short scale repulsion.

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Re: What is "touch" in molecular terms? @_@

Postby doogly » Sun Sep 20, 2009 2:39 pm UTC

Tass wrote:The pauli principle is indeed important. It forces the electrons into very antibonding orbitals as the nuclei get close.

Yeah, I think the Dyson, Lieb, Thirring (1975) analysis famously showed that Pauli was more important than Coulomb for stability of matter. I am not sure if this extends to touch, but if it doesn't, it is probably because there is some biology that is important.
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