Carbon-Carbon bonds in diamonds
A diamond crystal is made up of carbon atoms, each of which is
bonded to four other carbon atoms with single covalent bonds.
This leads to a question--how are the carbon atoms on the outer edges
of the crystal bonded? Obviously the geometry is different for them
and they cannot have the same sp3 hybrid configuration as carbon
atoms in the middle of the crystal. This question was asked of me by
a fellow high school science teacher so feel free to get technical if
I don't think you realize the implications of this question! I've asked
a professor of inorganic chemistry who is an expert on crystal structures
and I have yet to ask a p-chemist who is a surface studies expert. I'll
see if I can translate what they have to say to somewhat understandable
form :) I'll start with this. Don't get hung up on hybridization.
That's just a convenient way of expressing orbitals but it's more
complicated than combining s and p to get sp. Yes, the outer carbons are
different. But hybridization may not be the way to explain why.
I'll get back to this.
OK, I talked to the surface scientist and again to the inorganic prof.
Here's what they have to say: The carbons on the surface essentially
have an empty hybrid orbital. Since it's not bonding to another carbon,
it's definitely distorted (the surface prof called it a "dangling bond").
Possibilities as to what is going on at the surface include: delocalization
of electron density, bonding to either O or H (though O is unlikely since
oxidizing carbon leads to very thermodynamically stable carbon monoxide),
and bonding to another carbon that has the non-bonded orbital. I think I
I said empty orbital up there...I mean non-bonded. There's definitely
an electron there, it's just not bonding. I don't know of how much is
actually known about the surface of diamond since it's such a small fraction
of the total structure. Here are some references I dug up in the U of
"Adamantane: the chemistry of diamond molecules" Fort Jr.,R.C.
"Physical properties of diamond" R. Berman
"Properties and applications of diamond" Wilks & Wilks
"Properties of natural and synthetic diamond" J.E. Field
"Properties of diamond" J.E. Field
Hope this helps.
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Update: June 2012