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  1. #1 HELP!!!!!!! 
    h89
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    Hi,

    PLS I HAVE FEW (NAMELY 2) Q'S....IM GOING NUTS HERE! PLS ANSWER!!

    >>WHY IS THE MELTING POINT OF DIAMOND HIGHER THAN THAT OF SILICON OXIDE WHEN THEY BOTH HAVE SIMILAR STRUCTURES???

    >>THE Si-O BOND IS STRONGER THAN THE C-C BOND...WHY?!?!

    Pls dont ignore & answer! pls!


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  3. #2  
    Universal Mind John Galt's Avatar
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    The structures are similar, but they are not identical. For one thing the lattice density of diamond is very high (i.e. the atoms are very close together). That is suggestive.
    You might also wish to consider the effect of impurities, which are likely to be much higher in quartz than in diamond. (Though that's probably a second order issue.)


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  4. #3  
    h89
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    OK...but what about Si-O bond being stronger than the C-C bond...why?
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  5. #4  
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    1) The Si-O bond is quite polar . Quartz (SiO2) has a complex structure of interwoven helical chains, and is best thought of as a hard 'network solid'. In a liquid state, the partial atomic charges resulting from this polar bond will encourage multiple coordination between Si and O atoms, which allows disordered intermediate structures to form at relatively mild thermal conditions as the atoms slide about each other. Indeed, the barrier to bending a Si-O-Si bond is quite low, and the torsional barrier of bending a O-Si-0-Si bond is also very low.

    In contrast, non-polar C-C sp3 type bonds of diamond crystal do not readily allow for low-energy disordered states to occur. Hence, diamond has to be heated to a higher temperature for the vibrational motion of the C-C bonds to overcome the barrier to allowing individual C atoms to leave the ordered diamond lattice. The barriers for bending the C-C-C bond are rather high, and the torsional barrier for bending a C-C-C-C is rather high.

    My guess is that your teacher is probably trying to make you suggest that SiO2 is a liquid at room temperature. It isn't, rather it's a glass ('disordered solid state').

    "Lattice density", the explanation offered in an above post, is a derived result of the properties discussed in the first two paragraphs, and not a cause. Impurites can have big effects, but my guess is that your teacher wants to know what happens if the pure materials are compared.

    2) Bond strengths:
    C-C bonds are slightly shorter than Si-0 bonds (154 pm vs 161 pm). Usually shorter bonds are stronger.
    But, because the Si-O bond is quite polar, the Si-O bond is strengthened by the attraction of the positive-charged Si nuclei to the negative-charged O nuclei. The attraction between the partially-charged nuclei is strong because the nuclei are relatively close with the short Si-O bond length (not much longer than a C-C bond).

    Also, in addition:
    The C-C sigma bond has strong electron sharing in a sp3 - sp3 bond interaction,
    The Si-O bond has both a sp3-sp3 sigma bond component and a (d-p)-pi bond component (some electrons from the p-orbitals in the oxygen share with the unoccupied d-orbitals of the Si). The (d-p)-pi bonding component is one reason the Si-O bond is as short as it is, and probably adds some additional strength to the bond.
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  6. #5  
    h89
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    Thank you for ur reply
    but the isnt it the SO4 ion which has pi bonds? The molecule SiO4 has no pi bonds...

    waiting for ur reply
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  7. #6  
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    Quote Originally Posted by h89
    Thank you for ur reply
    but the isnt it the SO4 ion which has pi bonds? The molecule SiO4 has no pi bonds...

    waiting for ur reply
    Si-O can have a special hybrid type of *partial* pi-bond, the result of back-donation of some charge from p-orbitals on the O into empty d-oprbitals on the Si. This is called a (p-d) pi-bond (also called p-pi d-pi bond). This can occur when an atom has empty d-orbitals which are approximately the same size as the occupied p-orbitals of the donor atom. This contribution to bonding is why the Si-O-Si bond is not 109 degrees like typical sp3 bonds.

    This unusual kind of bonding is usually not covered in textbooks until the graduate level. This website has some discussion of it (in German) http://www.hermes-trismegistos.com/quarz.htm
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  8. #7  
    h89
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    Hello,

    Um do you any site which explains it all in english?
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  9. #8  
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    You can have google translate german -> english in the first article I linked.

    I found a good discussion of SiO2 properties, and a little small discussion of p-pi d-pi bonding in Si-O. Its says this type of hybridization, while there, is 'small' but important:
    http://www.foresight.org/Conferences...lett1/#SiObond
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  10. #9  
    h89
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    How can I ever repay you? Thank you so much...really
    Thanks for all the help once again!
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