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B How Easy To Pull Two Atoms Apart?

  1. Aug 1, 2016 #1
    I was thinking about the forces holding atoms together.

    Some materials seem very, very strong.

    But if we take thinner and thinner slices of any material it is easy to imagine that pretty soon no matter what material it is we'll have a slice so thin it would be difficult not to tear it.

    So what if we had a slice one atom thick, or deep, say graphene, maybe, would it be strong enough to hold itself together if we draped it over a finger tip? Would it fly apart if we blew on it? If we pulled it apart would we actually feel the strength of it, would it be that strong?

    Trying to find out I only discovered tables of bond energies showing, for instance, that tungsten has a bond energy of 849 kJ/mo, diamond 713 and MgO 1000. So that's obviously not the right direction for me to look.
  2. jcsd
  3. Aug 1, 2016 #2


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    You've found what you're looking for, you just haven't recognized it. Breaking every bond in a mole of MgO molecules requires 1000 kj, so simple division will get you to the amount of energy required to break the bond in a single molecule of MgO.

    (There is a subtlety in that you want the bond-dissociation energy instead of the bond energy for complex molecules, but that doesn't matter for the ones you mention above).
  4. Aug 1, 2016 #3
    No, I saw that much but couldn't credit it as legitimate because it means tungsten is tougher than diamond and magnesium oxide tougher than both, doesn't it?
  5. Aug 1, 2016 #4


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    It doesn't necessarily mean that. The toughness of magnesium oxide doesn't depend on the strength of the bond between the magnesium and the oxygen (very strong), it depends on how well the molecules of MgO hang together (not so much). It's easy to turn a chunk of MgO into MgO powder, but separating the magnesium and the oxygen is a very different proposition.

    Someone who knows more solid-state physics than I could probably say more.
  6. Aug 1, 2016 #5
    There is stuff in the world like spider webs and silk that have amazingly strong physical properties for lightweight organic material.
  7. Aug 4, 2016 #6


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    For a single molecule, you wouldn't feel it. For the bond energy of a single molecule, you need to divide by a mole. A mole is about 6.022*10^23, so the energy of a single molecule is very small in terms of Joules, much smaller than you can feel.
  8. Aug 4, 2016 #7
    Hmmm, a reply I posted isn't here. I must have made a mistake somehow and not posted it.. !

    I had meant to say that in light of the above let's leave MgO out of the question and consider only, say, carbon atoms.

    Let's imagine a simple string of carbon atoms and imagine we could get hold of it and pull it apart. How much force would it take?

    In the most general terms - something like a puff of wind? Would it spontaneously fall apart?

    That is my question.

    Regarding the bond strength and the division of it my problem is that I am not sure what the bond energy actually represents. I know that I am concerned with the attractive force between the atoms. We would be pulling against it. This is not the same thing as the bond energy, I believe, right?

    Lastly the quote about MgO molecules hanging together 'not so much' makes me wonder why MgO gets quoted as having the highest bond energy? Higher than tungsten and diamond. Probably, I suppose, because as I suspect bond energy is not a measure of how strongly held together things are.

    Or perhaps I am making an even more fundamental mistake. I am assuming diamond atoms - just carbon, right? - are strongly bound together and that's the reason diamond is so hard. Wrong?
  9. Aug 4, 2016 #8


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    Not outright wrong, but strength of bond is not the only thing involved. Consider a bungee cord - it takes a huge amount of work to break one, much than it would to break a length of thin steel wire. Yet something tied down with a loop of steel wire is much more solidly fixed in place than something tied down with bungee cord. The bungee cord stretches more, so if an assembly is lashed together with bungee you can deform it more easily (it's softer) than something lashed together with thin steel wire.

    Diamond is hard both because the carbon-carbon bonds are fairly strong and because they are very unyielding - you can't displace an atom in the crystal lattice very much at all without expending the full bond energy.
  10. Aug 4, 2016 #9
    I'm just talking about breaking the connection. Looking for a simple thing.

    Perhaps it is all much of a muchness and, either way, the force needed is so small a fly's wing could provide it.

    But if there's an appreciable difference let me ask for two answers:

    . What force to separate two atoms within a crystal lattice.
    . What force to separate two atoms not in a crystal lattice.

    And, it is a different question but I would like to know - why is the atom in the lattice harder to displace in this sense? I mean we are not trying to move it around in the lattice. We are simply breaking the lattice. I feel many crystalline structures break easily simply because they are crystalline - as you say, no 'bungee' give or play in them.

    And I nearly forgot 'using the full bond energy' - so you are saying the quoted bond energy figure is in fact the attractive energy that must be overcome and we can/should indeed divide that figure to get the numeric answer?
  11. Aug 5, 2016 #10


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    For a macroscopic material, there are many measurements of material strength, but if you are just pulling two atoms apart, bond strength should be sufficient. There is no crystal structure when you are talking about two atoms, and there is no elasticity.
  12. Aug 5, 2016 #11
    You need to be aware also that there is a difference between hardness and strength (as measured by ultimate tensile strength, for example).
    The strength is more directly related to your question about pulling apart atoms.
    Diamond has a very high hardness but not the highest tensile strength. Steel can have higher or comparable values. It is not easy to compare, as it seem to be a wide range of values for diamond.
    Glass fibers may have strengths larger than both steel and diamond.
    So my point is that it depends a lot on the structure and on what do you call "strength".

    For a chain of atoms the force may vary a lot too. A polymer chain can be quite strong and not be blown by a pale of wind.
    It depends on the type of bonds between atoms.
  13. Aug 5, 2016 #12


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    Atom-Atom bonding strength in solids depends on MANY factors. For example, look at graphite. Within a single material that is made up of carbon atoms, you have varying bonding strength. The bonding along the c-axis of the crystal is long and significantly weaker than the bonding along the hexagonal plane. So it is a lot easier to break apart graphite across the c-axis, resulting in graphite being soft and brittle.

    Yet, arrange these carbon atoms in a different way, and you get diamond!

    And has been alluded to in this thread, you have to be clear if you are simply talking about separating isolated molecules into separate atoms, or separating atoms that are within a solid lattice. There are major differences in those two situations. You simply cannot extrapolate the behavior of two atoms in an isolated molecule to the same atoms in a solid. As Phil Anderson has famously said "More Is Different".

  14. Aug 5, 2016 #13
    My fault for not specifying precisely enough, I guess.

    Let me specify a chain of carbon atoms. Nothing more.

    How much effort, force, whatever, to pull that apart?

    I would very much like to know the details of these differences in strength and hardness as related to structure. But first things first, what about simply pullng apart a chain of carbon atoms? If you want to make it perhaps a little more 'real' as best as my pathetic understanding can do, let's call it a chain of carbon atoms each with two H atoms, if that makes any difference because of 'structure'. CH3CH2CH2CH2CH3. Like that. But my first vision was merely two (unspecified) atoms floating in the limbo.
  15. Aug 6, 2016 #14


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    The wikipedia article on carbon-carbon bonds lists some figures for the strength of that bond in various molecules. 100 kcal/mole or ##4\times{10}^5## joules/mole is the right order of magnitude. That works out to about ##10^{-18}## joules for a single bond. This is a very small amount of energy, perhaps one hundred-millionth of what you experience from the thunderous wingbeat of a mosquito.
  16. Aug 6, 2016 #15
    It makes a difference if you are talking about covalent bonds or ionic bonds. Ionic bonds are much harder to separate than covalent bonds. For instance, it is much more difficult to separate sodium from chloride in table salt, than it is to separate two hydrogen atoms in molecular hydrogen. Ionic bonds are 100% electrostatic attraction. Covalent bonds have nuclear repulsions to consider.
  17. Aug 6, 2016 #16


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    Pardon my interruption, but I've been following this thread for a couple of days, trying to solve it for myself, and have run into an independent problem.
    abrogard, in the OP, started by asking about the force required to break the bonds
    , and then mentioned
    force ≠ energy

    I didn't notice this at first, but this morning, I found a place that had all the numbers I was looking for, stating that a 4 kg cat could be held up by a 1 m2 sheet of graphene, weighing 0.77 grams.
    Doing the maths, I determined there were 3.87e19 carbon 12 atoms in the sheet, yielding 6.22 billion atoms to the side, which all have to break, yielding 6.31e-9 newtons per bond.

    Looking over the answers so far, most everything seems to be "energy" related. But the "cat in a hammock" claim, says it's a force that breaks things.

    This has me scratching my head.

    ps. Ok to delete, and tell me to start my own thread, as I've saved my post. :angel:
  18. Aug 6, 2016 #17


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    They're related. Work = force times distance, so if you apply enough force to increase the distance and keep pulling until something breaks, you'll have the total bond energy. A weaker force over a longer distance implies a softer material for the same bond energy.
    (But as ZapperZ's and nasu's answers above suggest, there's a lot more to it and I am seriously oversimplifying).
  19. Aug 6, 2016 #18
    Thank you. Job done. I did google but failed to google that particular entry, the very one I wanted.
    And you mentioned dissociation energy in about your first post, I think, I could have googled with that to good effect and didn't.

    You're anything but nugatory, aren't you?

    One hundred-millionth of the wingbeat of a mosquito! So such bonds freely forming in the atmosphere, say, would fall apart just as readily as they formed it seems, driven asunder by mosquito wingbeats and such, Is my current take on it. Only as they form en masse do other factors come into play that allows the agglomeration to persist.

    This suggests to me that if it were not for the factors arising from the mass of a substance the top layers would be constantly drifting off - and in fact maybe they are? But atoms are so tiny they can drift away continually one at a time for the longest time without appreciably diminishing the mass?

    Should be a different thread I guess.

    Scratch the surface of anything at all and a wealth of information and further question arises. It would probably take me a year or something just to comprehend the first layer of the Wiki article on carbon bonds.

    No wonder I never become expert on anything but just remain eternally an ill informed dilettante. :)

    My thanks to all who tried to come to my assistance.
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