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Giant covalent structures: how do they "end"?

Discussion in 'Science' started by ceviche, May 24, 2011.

  1. ceviche

    ceviche New commenter

    A question I remember wondering myself at school that I never got a satisfactory answer for. Now a student of mine is asking the same thing. How do things like diamond and silicon dioxide with their very ordered tetrahedral arrangements "stop" at the surface? I know these things have defects and it's not so simple as "the whole thing is just one big tetrahedral net", but how indeed does the net "end"?
     
  2. ceviche

    ceviche New commenter

    A question I remember wondering myself at school that I never got a satisfactory answer for. Now a student of mine is asking the same thing. How do things like diamond and silicon dioxide with their very ordered tetrahedral arrangements "stop" at the surface? I know these things have defects and it's not so simple as "the whole thing is just one big tetrahedral net", but how indeed does the net "end"?
     
  3. the structure keeps going until the edge where it stops. But, the surface represents a region that is atypical of the bulk and will be the result of reactions with whatever the surface happens to in contact with at the time. In metallic structures the different faces will have potentailly different arrangeemnets of metal atoms exposed depending on which plane is exposed. This can result in some interesting differences in reactivity between these sites on the same "surface"..
    Remember that the reduction potential of aluminium is really quite negative, yet the "metal" reflects little of that behaviour because the surface is NOT aluminium rather the normal oxide. By the time we get hold of it it has ALREADY reacted in the way you'd expect for a couple with negative Eo value.
     
  4. I suppose a similar problem concerns what happens at the ends of an addition polymer.
    Perhaps our understanding of bonding etc is a little too simplistic.
     
  5. ceviche

    ceviche New commenter

    Some info on chain termation: http://en.wikipedia.org/wiki/Chain_termination
    Thanks,
    aluminium is a good example as are the alkali metals, as the oxide
    layer is mentioned in our teaching. That's fairly straightforward
    though, I was more concerned with giant covalent structures like quartz,
    silicon and diamond. It makes sense of course that the surface structure is different and is the result of chemical reactions, most likely with water and oxygen.
     
  6. One of the things that was so surprising about the discovery of graphene, a monolayer of graphite, was that it has turned out to be so stable. Generally everything is completely different for the boundary layers.
     
  7. The way I tend to think of it is that giant structures aren't limited by the number of atoms present ie CH4 will always only have 5 atoms: what limits the size of a diamond/sodium chloride etc is the physical size of the crystal rather than chemical proportions - hope that helps :)
     

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