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General Science => General Science => Topic started by: deweys hamster on 12/08/2004 20:21:30

Title: What's at the edge of a crystal?
Post by: deweys hamster on 12/08/2004 20:21:30

Hello everyone,

I always wanted to know what was bonded to the outermost carbon atoms in diamond.  I can see that the strength of the structure comes from the covalent bonds to four other carbon atoms that would happen within the crystal, but what are the very outer carbons bonded to so that the valency is satisfied?  What happens when the crystal is cut/broken so that inner carbons are separated from their neighbours?
[:)]

Title: Re: What's at the edge of a crystal?
Post by: qpan on 13/08/2004 09:23:03

Hydrogen atoms are bonded to the remaining free bonds of each carbon on the outer edge. When the crystal is cut, atmospheric hydrogen (i guess) bonds to the carbon, but if there is no hydrogen present in the atmosphere, not sure what would happen!

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

Title: Re: What's at the edge of a crystal?
Post by: deweys hamster on 18/08/2004 21:37:38

....ok, i can accept hydrogens for covalent bonds.
what about ionic crystals then??  e.g. NaCl could have protons bonded onto the outer chlorides, but what would be on the sodiums?

Title: Re: What's at the edge of a crystal?
Post by: qpan on 19/08/2004 09:42:08

Well, not entirely sure about ionic crystals- but couldn't hydrogren still be bonded to the outer edge atoms? Hydrogen could bond ionically with Na and covalently with Cl.

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

Title: Re: What's at the edge of a crystal?
Post by: qpan on 19/08/2004 13:53:03

No wait- just thinking about it- there's no actual need for anything to be bonded to the outer edge as all the positive and negative ions are paired up, so there are no spare bonds. A single molecule of NaCl would happily exist without external molecules needing to bond to it due to all valancies being taken up.

"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe

Title: Re: What's at the edge of a crystal?
Post by: Ylide on 20/08/2004 08:24:24

You really can't compare diamond and ionic crystallize structures.  The lattice of diamond is all covalent bonds...diamond is just a huge molecule of covalently bonded carbon.  I'm not entirely sure how it fills out its valence shells.  Graphite is different in that it's made of planar rings of carbon.  The bonds within the ring are covalent, but the attractions between the planes are rather weak, allowing the planes to slip.  (giving graphite its interesting properties)

Ionic crystals, though, are held together by opposite charges.  For instance, NaCl is a cubic array of alternating Na and Cl molecules.  As gpan said, they're always paired so the ones on the outer edges don't have any unpaired electrons.  The attractive force between each NaCl molecule is an ionic bond.  The attractive force between NaCl molecules is a polar attraction, not very strong at all.  Generally speaking, the greater the dipole moment of the molecule, the stronger its crystal lattice is going to be because the intermolecular attractive forces will be stronger.



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