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Help with 'elementary' question

  1. Jan 16, 2005 #1
    'Silicon and phosphorus are both covalent substances, but silicon has a much
    higher melting temperature than phosphorus'

    Explain this statement.


    That was the question..im a bit stuck on it. I dont usually ask here, but im out of ideas for this for now..

    I first thought of the electronic configuration of the two elements, with Si having 2p2 and P having 2p3.

    I first thought the number of molecular orbitals might be larger for Phosphurous, but found out the question was asking the reason for the opposite! doh!

    If you need more detail for my effort put into this question, ill gladly post.. however, i need some help with this question. Thanks in advance to those who help.

    EDIT: i think it has to do something with the shape and structure of the molecules the atoms form when they covelently bond with other atoms. Im off to research, but thanks to anyone who helps..
     
    Last edited: Jan 16, 2005
  2. jcsd
  3. Jan 16, 2005 #2

    chem_tr

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    If there are only phosphorus or silicon atom, then they will have to combine with themselves, so silicon has the ability to form tetrahedral or square-planar geometry as carbon does with structures we know (respectively, diamond and graphite). Obviously, the bonding ability comes from the available electron configuration, so phosphorus has one additional electron, which prevents them making a very stable bonding as silicon does. This is why phosphorus has much weaker bonds than silicon.
     
  4. Jan 16, 2005 #3

    Thanks for the quick reply! o:)

    Just another question. Since you said silicon has more stable bonding, does that mean the tetrahedral shape is more thermodynamically stable than bonds formed by phosphorus?

    I ask this because carbon structures, such as graphite, have more thermodynamically (but not physically) stable structure than the other structure diamond (tetrahedral).
    So therefore can i conclude,
    'tetrahedral bond structure is thermodynamically LESS stable than a sheet bond structure (of graphite) but it IS more stable than the phosphurous bonding structure (trigonal bipyramid?)'
     
    Last edited: Jan 16, 2005
  5. Jan 17, 2005 #4

    chem_tr

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    I am not sure what kind of shape will phosphorus atom show in bonding, but it is obvious from the configurations that the shape is not tetrahedral, so it is not as stable as a tetrahedral framework.

    I don't agree with you about thermodynamical stability of graphite towards diamond, graphite structure may be kinetically favored, but eventually diamond is much more stable (thermodynamic). However, treat this still a discussion, I have no certain knowledge about it.
     
  6. Jan 17, 2005 #5
    I think bond energy is also concerned. Silicon compound bonds in a network covalent structure which has extremely high bond energy while phosphorus does not. This means that breaking intermolecular bonds requires more energy, and thus the higher melting point.
     
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