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Valence Bond Theory

Bonds result from the overlap of atomic orbitals.

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Credit: Ben Mills
Source: http://commons.wikimedia.org/wiki/File:Bis(benzene)chromium-from-xtal-2006-3D-balls-B.png
License: CC BY-NC 3.0

Ever since Kekulé had his dream about a snake swallowing its tail (or so the story goes), there has been discussion about the exact structure of the benzene molecule. This simple compound is composed of six carbon atoms and six hydrogen atoms, but how the carbons are linked together was a problem. Reactivity studies indicated a different structure than might have been expected if there were simply alternating carbon-carbon single bonds and carbon-carbon double bonds.

Amazing But True

  • We have known for decades that we cannot specifically locate an electron in space. All we can say is that it is possibly somewhere in this general region. OK, so that worked for a while as long as we just talked about atomic orbitals. What we need to keep in mind is that the atomic orbital is a mathematical construct and not necessarily the real thing.
  • Credit: Kurzon
    Source: http://en.wikipedia.org/wiki/File:Neon_orbitals.JPG
    License: CC BY-NC 3.0

    The shapes of the first five atomic orbitals [Figure2]

  • But then we started to get detailed structural data on compounds where the atoms are connected by covalent bonds. Our ideas about covalent bonds needed to change significantly. In a compound like methane, the carbon atom model of two 2s and two 2p electrons doesn’t work anymore. So now what? Well, we design a new model. We talk about electrons being promoted to new energy levels and being hybridized. But are they really?
  • To date, there has been no visualization of these hybrid orbitals. We are left with the assumption that they exist because it allows us to explain why we get the bond angles we do in different compounds. The reactive properties of benzene make perfectly good sense using the idea of hybrid orbitals. But we need to keep in mind that this model is just that – a model. Do electrons really occupy the spaces we say they do? The math says “Yes”, but the mind still wonders.
  • Watch a video about hybrid orbitals at the link below:https://www.youtube.com/watch?v=SJdllffWUqg

Show What You Know

Use the links below to learn more about hybrid orbitals. Then answer the following questions.

  1. What are atomic orbitals?
  2. What does valence bond theory deal with?
  3. Where is the charge density in a pi bond?
  4. Describe the acetylene triple bond.
  5. What did the last web site say about hybridization?

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