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Hybridisation question

  1. Nov 11, 2014 #1
    please answer.QUESTION ABOUT HYBRIDISATION. - during determining hybridisation we look only for central atom ,other atoms with which central atom combines ,say for eg ,in co2 we take c as central atom and take its hybridization but does oxygen also hybridized before forming sigma bonds?
     
  2. jcsd
  3. Nov 12, 2014 #2

    DrDu

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    In oxygen, the energetic splitting between the s and the p orbitals is much larger than in C, so hybridization is inefficient. The valence orbitals on O will be p orbitals while the s orbitals will stay doubly occupied and not participate in bonding.
     
  4. Nov 12, 2014 #3
    what do you mean by doubly occupied ?
     
  5. Nov 12, 2014 #4

    DrDu

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    two electrons
     
  6. Nov 12, 2014 #5
    ok you are saying as hybridization to occur energy difference between orbitals should not be large as in oxygen but if we consider H2O (water molecule) in this central atom is oxygen ,so there will be hybridization of oxygen but energy gap will be still same as in oxygen of co2 .so why hybridization of oxygen is possible in H2O but not in CO2?
     
  7. Nov 12, 2014 #6

    DrDu

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    You don't need hybridization to explain the bonding in H2O. Each hydrogen can make a bond with one p orbital.
     
  8. Nov 12, 2014 #7
    but hybridisation explains shape of H2O which is bent.
     
  9. Nov 12, 2014 #8

    DrDu

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    There is also an angle between two p orbitals. Ok, it is a little bit smaller than the bond angle, but this angle between the bonding orbitals on O is supported by actual calculations.
     
  10. Nov 12, 2014 #9
    so you mean in H2S also hybridization of sulpher is not there ?then why my textbook explains all these on the basis of hybridization.
    /
     
  11. Nov 12, 2014 #10

    DrDu

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    With sulphur, the basis for hybridization is even thinner. The bond angle is smaller and the s and p orbitals are of quite different size, so that hybridization won't be effective.

    I don't know why text books perpetuate stuff which has been proven wrong for more than 50 years. I suppose because chemistry teachers usually have only a faint idea of theoretical chemistry.
    Specifically the principle of maximal orbital overlap introduced by Pauling is fishy.

    My advice is the following:

    1. Don't expect too much from qualitative models. Predicting a bond angle precisely is very difficult.

    2. Valence bond theory tries to explain the electronic structure of molecules using free atoms as a starting point. So use the normal s and p orbitals where possible.

    3. In some cases, specifically carbon compounds, you have to start from an excited carbon atom with the s and the 3 p orbitals occupied with one electron each. This is called "promotion".
    If you don't do this, you will have difficulties to explain why there are 4 bonds instead of only two.
    Then also hybrid orbitals can be used.

    4. Never invoke d-orbitals (or spd hybrids) to explain bonding in main group elements.
     
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