What's holds matter together?

[rthmjohn]

This may sound kind of dumb, but what exactly holds things together? I know that in ionic compounds like carbonates, the ionic bonds keep the individual ions in a regular formation and when you break a block of NaCl, for example, in half you are simply breaking the ionic bonds. But what about things that aren't ionic? When you tear a piece of paper, what exactly is happening? Why can't you "put it back together" so to speak? What about plastics? When you stretch them to the point that that they rip or break, what is happening at the molecular level? What was holding the molecules together in the first place? And metals... when you have, say, a sheet of copper what is holding the atoms together. Are the atoms turned to cations with their electrons acting as a "glue." What happens at the atomic level when you weld to pieces of metal together and why isn't it possible to simply put them together like puzzle pieces? Sorry if I sound stupid, but I really am clueless as to how these things happen.

[RyanJ]

Actually its an interesting question. When you rip paper you can't be tearing throught the middle of an atom so you must be breaking some bonds.

 

These bonds are between atoms or groups of atoms that share electrons as such when you pull them appart you basically get a set of ions

 

The thing is, like magnets, negative and positive like to be neutral and so attract one and other and in dound so form bonds. When something is solid it is then said that the temperature of the air is insufficient to break the bonds of the element or compound. So, I suppose you could say it does act like a big jigsaw puzzle

 

Cheers,

 

Ryan Jones

[YT2095]

you must be breaking some bonks.

 

sorry, but that was just too damn funny to let go

 

as for the original question(S) there`s an answer to each of them, but way to many all in one go for me, basicly it still all comes down to the same thing of attraction on an atomic level, you cover everything from polymers to metals and others in that post, but it`s all about Sharing Charges and ending with a Stable product.

 

Ryanj, you`ve just made my day, and cheered me up no end

[RyanJ]

Ryanj' date=' you`ve just made my day, and cheered me up no end [/quote']

 

My spelling is real bad, you'll probably end up seeing loads of those

 

Cheers,

 

Ryan Jones

[woelen]

Many covalent solids and liquids are kept together by so-called vanderwaals forces. These are attracting forces between molecules and atoms.

 

So, if you tear apart a piece of paper, or a solid covalent crystalline compound like sugar, then you are not breaking molecular bonds, but vanderwaals forces between molecules. These forces only are relevant at very low distances (at the molecular level) and they are the result of a delicate second order balance of electrostatic forces. The reason for the existence of vanderwaals forces is that, although molecules as a whole are neutral, the charge density is non-zero (positive in nuclei, negative in the electron clouds).

 

Just as an exercise, think of the following system:

 

P--N.......................................P--N

 

Here, P and N are charges of the same magnitude, but with opposite sign. Now suppose P--N is a rigid stick, with P and N having a distance d. Now, also suppose that the distance with ............ equals r. You'll see that if you compute the force between the two sticks (purely electrostatically), that this is non-zero, if d is not equal to 0, although the total charge of each stick equals 0. This is a very simple model, but it does demonstrate the basics of vanderwaals forces.

You'll also see, that for r much larger than d, the vanderwaals forces are amazingly small. If you do the math, then you'll see that they fall off with r^4.

 

The reason why paper and so on do not stick together again is that when you put the pieces next to each other, only at very few points you manage to bring the molecules sufficiently close to each other that there are relevant vanderwaals forces. In theory, when you have two very smooth surfaces, and you bring together these two, they would stick together. Certain animals use this effect, such as certain gekko's. They can walk upside down, even on smooth metal and glass surfaces. Flies also can do that, but they use vacuum between their legs and the surface they are walking on. Gekko's, however, are too heavy, relative to their contact-area and they use the stronger effect of vanderwaals forces.

[The Thing]

Down in the individual atoms there is still the nuclear strong force (residual strong force that acts upon hadrons: mesons and baryons) acting upon the nucleus to keep it all together. Else the atom will just explode with protons flying everywhere away from each other.

[Xyph]

Gekko's, however, are too heavy, relative to their contact-area and they use the stronger effect of vanderwaals forces.

I've always heard that geckos can cling to apparently smooth surfaces like glass by gripping tiny imperfections, invisible to the naked (human) eye, with microscopic hairs on their feet... Is this incorrect? If geckos can use vanderwaals forces to cling to glass, wouldn't that mean that you could make 2 panes of glass stick together?

[The Thing]

Geckos do cling to surfaces using Van der Waal forces like woelen said. They use tiny microscopic hairs called setae, and on top of each seta there is a tiny pad called the spatulae. And when this pad comes in contact with surfaces the molecular forces (van der waal) between the molecules in the spatulae and the molecules on the surface gives the gecko more the sufficient forces to hang onto the wall. The gecko then peels the pads off like tape, and keeps on crawling.

[woelen]

I've always heard that geckos can cling to apparently smooth surfaces like glass by gripping tiny imperfections, invisible to the naked (human) eye, with microscopic hairs on their feet... Is this incorrect? If geckos can use vanderwaals forces to cling to glass, wouldn't that mean that you could make 2 panes of glass stick together?

What you say is correct. These microscopic hairs on their feet are so thin and numerous, that they provide sufficient vanderwaals force to keep the animal sticking to the surface. Currently research is done with nanotubes on a substrate, which could be put in shoes and gloves, allowing trained humans to 'walk' on vertical walls, glass, etc.

 

As I stated in my previous post, in theory, when the surface of two smooth panes of glass is really really smooth, it would be possible to stick them together. But even the smoothest glass has nano-scale wrinkles, irregularities and so on, such that this prevents to two panes from sticking to each other. The gecko's feet, however, have hairs, which perfectly adapt to all these irregularities and that is why these animals can walk on such surfaces, even upside down.

 

So, the gecko's do not grip the imperfections of the surface, but their littles hairs snugly fit into these imperfections. Just, because of these little hairs fitting well in all kinds of imperfections, allows these animals to walk on virtually every surface, beit smooth or rough.

[jdurg]

I've actually experienced this between two sheets of gold metal. In my collection, I have numerous samples of 24 karat gold in the form of 1 gram slabs. The backs of these slabs are ULTRA smooth, and if I place the smooth side of one slab next to the smooth side of another slab, you can actually feel them 'stick' to each other as it takes a bit more force than normal to move them apart.

[Xyph]

Ah, okay, thanks for explaining that. Thinking about it now, I suppose it would be pretty difficult to actually cling onto imperfections with such tiny hairs if there weren't another force allowing the hairs and the surface to stick together.

[xeluc]

I've actually experienced this between two sheets of gold metal. In my collection, I have numerous samples of 24 karat gold in the form of 1 gram slabs. The backs of these slabs are ULTRA smooth, and if I place the smooth side of one slab next to the smooth side of another slab, you can actually feel them 'stick' to each other as it takes a bit more force than normal to move them apart.

 

I have also experienced this happening, not with Gold slabs though. I always thought it was a suction of some sort since the to things were so smooth. I now know differently, Interesting.

[RyanJ]

I have also experienced this happening, not with Gold slabs though. I always thought it was a suction of some sort since the to things were so smooth. I now know differently, Interesting.

 

I've also had something simmilar, once with a piece of really smooth [acr=Polytetrafluoroethylene]PTFE[/acr] and once with Silver metal

 

Cheers,

 

Ryan Jones

[rthmjohn]

But what about metals whose nuclei are in regular arrays and whose electrons form a "sea" around them? Is this an example of ionic bonding? Does van der waals forces explain how the molecules in a clay pot or a wine glass are held together?

[RyanJ]

But what about metals whose nuclei are in regular arrays and whose electrons form a "sea" around them? Is this an example of ionic bonding? Does van der waals forces explain how the molecules in a clay pot or a wine glass are held together?

 

It could be a combination of both, each can effect the structure in its own way, the ionic bond is strong but many van der waals forces are strong enough to make a difference also! So its depends

 

Cheers,

 

Ryan Jones