# What holds solid matter together?

1. Mar 8, 2011

### Jarfi

When you have a cube made of any matter plastic, iron ,wood it is made of millions of atoms.
But those atoms are held tight in the same shape. Why don't the atoms just fall away from each other and change in to a very fine dust of molecules. What holds all these different elements together?

I would think electric forces but there is both + and - so they cancel each other out and there is no attraction force anymore.

I know that molecules are are hold by different charges of the atoms, but I learned in school that the other kind of molecules is not hold together by different charges?? than what holds them together.

And what holds solid matter together, what force?

2. Mar 8, 2011

### Staff: Mentor

The electromagnetic force is what holds matter together. It is actually a little complicated. The electromagnetic force isn't completely neutral between atoms. There is usually a residual force resulting from an array of quantum phenomena such as the Pauli Exclusion Principle, the Van Der Waals force, and several others. The pauli exclusion principle is what we call the the observation that electrons cannot be in the exact same state as another electron, but in discrete orbital states. This keeps multiple electrons from piling up all on each other and is responsible for matter being "solid". I'm a little less educated on the Van Der Waals force, so I recommend hitting up Wikipedia or Google to learn more.

3. Mar 8, 2011

### RedX

What's weird is that the electromagnetic force seems to tend to be attractive.

If you rub your hair with a comb, the comb will then have the ability to attract neutral objects such as small piece of paper.

Although I've never seen it it before, I think the hair will also attract small pieces of paper.

I think this phenomena is van der waal's force, that random fluctuations in the distribution of charge within a neutral object, results in something attractive.

Something like the covalent bond I believe is pretty much understood in terms of the Coulomb force, but the key seems to be that you have to use the Coulomb potential in a quantum mechanical equation (the Schrodinger equation), so it's not pure electrodynamics (but the correct potential to use seems to be pure electrodynamics). Then again the atom is not stable under pure electrodynamics, so it shouldn't be surprising that you can't describe bonding in terms of just electrodynamics. So the electromagnetic force holds matter together, but in a quantum mechanical way.

As for metallic bonds I have no clue how that works. The idea seems to be that a sea of electrons acts as a glue that holds together an array of nucleons. Not sure how Schrodinger's equation gets you that.

Also ionic bonds I have no clue.

Actually, now that I think about it, I don't believe chemical bonds have anything to do with a Coloumb potential at all. It seems to be just a consequence of quantum mechanics.

If you have two individual particles, then the Hamiltonian can be written as a 2x2 matrix:

$$H_{11},0$$

$$0,H_{22}$$

If you introduce a transition then the matrix is:

$$H_{11},H_{12}$$

$$H_{12},H_{22}$$

where for simplicity I am taking all values real (otherwise there would be a conjugate in one of the diagonal elements).

Anyways, I think the idea is that this second matrix will have an energy state that has an eigenvalue lower than .5(H11 + H22). The lower energy level is called a bond, the higher level an anti-bond. Both particles will be in the bond, so that the total energy would be lower than 2*.5(H11 + H22), which is just the sum of the energies without bonding, or (H11 + H22). This difference is the binding energy.

Or something like this. Anyways, something like this I think can explain why chemical bonds produce less energy: has nothing to do with attraction at all it seems.

Last edited: Mar 8, 2011
4. Mar 8, 2011

### Staff: Mentor

The electromagnetic force is only attractive between oppositely charged particles. For like particles it is repulsive. This is exactly why matter repels other matter and doesnt let it pass through. As the electrons get closer to each other they feel repulsion. Enough to make things solid.

5. Mar 8, 2011

### RedX

Oh I get it. The reason that the electromagnetic force tends to attract rather than repel is because the force decreases as distance increases. If instead of a (1/r)2 potential, you had an r2potential, then the electromagnetic force would tend to repel. So the asymmetry is not caused by charge, but by separation.

Anyways, going back to quantum mechanics, the reason I guess why atoms attract each other is because the electrons then have more space: they can spend time near the nucleus of the other atom and not just their own. In other words a superposition of shells surrounding both nuclei offers lower energy than each electron staying in their own individual nucleus. Maybe this can be interpreted as more freedom to roam around equals less energy, since Heisenborg's uncertainy principle says that the larger the space the smaller the momentum and the smaller the momentum the smaller the energy.

So maybe the reason atoms attract each other is Heisenborg's uncertainty principle?

6. Mar 10, 2011

### A. Neumaier

No. Then the universe would be a huge black hole. It is the sign of the potential that makes the difference between attraction or repulsion. the r-dependence only tells how strong it is at various distances.

7. Mar 10, 2011

### _PJ_

Electromagnetic force maintains the chemical bonds in molecules.
The 'cancellation' refers to the system as a whole. The system of a + and - overall has 0 electromagnetic charge, however, the + attracts the - and the - attracts the + equally.
I think you might be referring to the difference between ions and covalent bonding. WIth ions, a negatively charged atom - an Anion (one that has 'gained extra electrons') is electromagnetically attracted to a positively charged atom - a Cation (one that has 'lost' an electron)
With Covalency, electrons seek to form pairs and fill the orbital shell. This results in a stable energy levell and so is most 'desirable'. Atoms can "share" electrons to accomodate this fulfilled state mutually.
This is STILL electromagnetic, since the energies transferred and used to make these bonds are all transmitted via photons between electrons.
---
It is true, that Quantum uncertainty is a factor since when two atoms are in proximity, the uncertainty in the outer electrons' positions can dissassociate their identity as 'belonging' to a particular atom, but the relation between an electron and a nucleus is still electromagnetic attraction.

8. Mar 10, 2011

### RedX

I wasn't very clear, but I was wondering how something like attraction could be favored over repulsion when dealing with neutral particles. That two neutral atoms attract, say under a 12-6 potential, is due to decreasing strength at increasing distance. The simplest case to consider would be if the electromagnetic force didn't depend on distance - then neutral objects would never attract or repel.

Anyways, this gives me an idea to stop the expansion of the universe. If you were to put charges into space, alternating between negative and positive charges, then wouldn't that slow down the expansion of the universe? Or would that not work? It seems to work to hold a solid together, so why not the universe?

9. Mar 10, 2011

### Jarfi

So you mean that the atoms all share a sea of electrons and attract to this shared cloud of electrons in solids? Wasnt it only in metals?, i am asking how can two atoms both with overall 0 charge be held together. What force in solid matter where all atoms are the same holds the object made out of the atoms together without becoming liquid or gas?

10. Mar 10, 2011

### Staff: Mentor

Hrmm. The best I can answer is that at the quantum level, the different charges aren't 100% cancelled out in an atom at close range. This is due to varying energy levels in the electron shells and the fact that electrons are effectively "spread out" all over the atom. Until you reach a full configuration, such as in the noble gases, the atoms want to give up or gain electrons to reach that configuration. This is why elements on the left side of the periodic table are so reactive. They are only 1 electron away from a noble gas configuration. Same for the ones all the way to the right side, next to the noble gases. They only need 1 electron to reach that configuration.

11. Mar 10, 2011

### Staff: Mentor

Nope. It is SPACE itself that is expanding, not the matter that makes everything up. Every point in space is expanding away from every other point in space. Imagine blowing up a baloon. The surface of the baloon represents space itself.

12. Mar 10, 2011

### RedX

That's what I thought too. But there's this argument that the expansion should slow down because gravity should deaccelerate the expansion.

It doesn't make sense to me because I too have the picture of the balloon, and that space itself is expanding. But if people are going to use the argument that gravity should deaccelerate the expansion, then I thought I'd just throw in why not use the electromagnetic force. Already neutral objects attract each other, although very weakly it goes like 1/r7 or what you use for the van der waals force, which is much weaker than 1/r2, so if gravity can't stop the expansion I doubt van der waals can.

I just thought a solid resists being pulled apart, so if you make the universe one big solid or something you could slow it down.

13. Mar 10, 2011

### Staff: Mentor

Hrmm. You make a good point Redx. I don't know how gravity affects spacetime, other than the belief that gravity its simply curved spacetime.

However, I do know that current evidence points to the acceleration of the expansion of space. This means that gravity will NOT pull everything back together.

14. Mar 11, 2011

### A. Neumaier

No. It is due to the sign of the interaction. Changing the sign but keeping the 12-6 law would make the interaction attractive at short distance and repelling at large distance, with negligible effect far away.

15. Mar 11, 2011

### Naty1

There are four fundamental forces....electromagnetic, strong and weak, and gravity.

Gravity doesn't affect the structure of matter much because it is billions of time weaker than the other forces. The weak force accounts for radioactive decay and affects only radioactive material.

That leaves the strong force which binds he components of nuclei together, and the electromagnetic force which bonds negatively charged electrons to positively charged nucleii...it's the force that holds electrons in the vicinity of a nearby nucleus.

Solid matter is 99.9 % empty space...between electrons and nucleii for example....like the space between the earth and the sun.

Different combinations of atomic components have different structures and characteristics: for example conductors of electricity have some loosely bound outer electrons which move rather easily when an electric potential is present....copper and gold are examples. Others will not release their outer electrons easily...these are insulators, like plastic,rubber or ceramic. Acids behave one way, alkalines another....

More details here:

http://en.wikipedia.org/wiki/Matter

and here:

http://en.wikipedia.org/wiki/Crystal_structure

Last edited: Mar 11, 2011
16. Mar 11, 2011

### A. Neumaier

More correctly, solid matter is 99.9% space filled by the electron field, a very low-density glue-like substance. See the entry ''Does an atom mostly consist of empty space?'' in Chapter A6 of my theoretical physics FAQ at http://www.mat.univie.ac.at/~neum/physfaq/physics-faq.html#touch

17. Mar 11, 2011

### _PJ_

Again, it's only the overall resultant charge that is (fluctuations aside) "cancelled out". Within the scale of atoms, continuous photons transmitted between particle repelling and tattracting cause the molecules to vibrate. Occasional quantum fluctuations as well as well as external influences may break these bonds, depending on their stability and the energy of the external influence.

This is why gravity dominates the cosmological scale, since being always attactive, the energy is always positive for gravitational attraction, there's no negative to cancel it out (ignoring Dark Energy theories for now)
Though even small electromagnetic charges can counter gravity due to the immense strength difference of the forces. It's possible for a table to support a brick with its electromagnetic structural integrity, despite the gravitational pull.

The expansion of the univferse could be slowed by gravity, provided:
a) There are massive objects close to the boundary of the universe.
b) The gravitational attraction would be stronger than the pressure.

IT is nonsensical to consider using electromagnetic energy, since the 'fabric of the universe' has no electromagnetic cahrge.

18. Mar 11, 2011

### Jarfi

So it's atoms with loose electrons that tend to stick together, atoms that can share electrons?

19. Mar 11, 2011

### shoestring

For a single atom, the positively charged nucleus can be thought of as creating a potential well for the electrons. Thanks to quantum mechanics, there'll be a set of discrete energy levels available for the electrons.

In a similar manner, a large metal gitter can be thought of as a wide potential well for the valence electrons, and the width of the potential well makes the energy levels much closer spaced than in a single atom. Perhaps this increase in availability of lower energy levels is a reason why it's energetically favorable for some materials to form large aggreagates.

20. Mar 11, 2011

### A. Neumaier

inert gas atoms have no electrons to share. Loose electrons are easily lost, giving positively charged ions. Atoms with ''holes'' in their outer shell easily pick up these loose electrons, giving negatively charged ions. But if they get close they share things and their electron shells fill up - this causes the binding.