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Why do we draw unbonded electrons in pairs?

  1. Apr 19, 2015 #1
    A little background: I'm only a highschool student with some knowledge on Lewis dot structures. And I don't know much about the s orbitals or p orbitals or whatnot.

    Why are there lone pairs? Shouldn't the electrons repel each other? Why do we draw them as pairs?

    For example: carbon dioxide is drawn like this
    CO222.png
    Why don't we draw it like
    CO22.png
    or
    CO2.png

    or sulfur dioxide
    SO2.png
    Why not this?
    SO22.png

    Are the drawings just arbitrary and the original used for simplicity's sake?
    Can all these drawings work?
     
    Last edited: Apr 19, 2015
  2. jcsd
  3. Apr 19, 2015 #2

    Simon Bridge

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    Electrons couple spin-spin inside their orbitals - thus: they tend to pair up. Inside a molecule, electrons are not static charges so all the diagrams are incorrect. However, if you work out the relative frces in the different configurations you drew, you'll find your drawings have the higher potential energy.

    It's only a notation - not an accurate representation of the distribution of electrons in the molecule.
     
  4. Apr 19, 2015 #3
    I don't quite understand. I read http://chemwiki.ucdavis.edu/Physica...Quantum_Number_.26_(s.2C_p.2C_d.2C_f)_Orbital and to my understanding a pair of opposite spin electrons can occupy an orientation of an orbital? Is that why we say they are pairs?
    So all those drawings are correct? I can draw that way if I wanted to?
    Also, what notation would be an accurate representation?
     
  5. Apr 20, 2015 #4
    you can not do that,it is wrong.
     
  6. Apr 20, 2015 #5

    DrDu

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    At least for elements from the first period (Na - Ne), there are only four valence orbitals available 1 s and 3 p type orbitals. Taking oxygen in CO2 as an example, you can form two bonding orbitals with C and two anti-bonding orbitals, which are too high to be occupied. The bonding orbitals are filled with two electrons from C and two electrons from O. This leaves 4 electrons on O which have to be distributed among 2 remaining orbitals. As each orbital may contain at most 2 electrons with anti-parallel spin ("pairs"), the only possibility is to have two pairs at O while the structures you did draw aren't possible.
     
  7. Apr 20, 2015 #6

    Simon Bridge

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    Yes. One spin up and one spin down.
    This should hep you make sense of the convention in pairing up the dots. An unpaired dot would indicate would indicate an electron all by itself in the "orbital" - two unpaired dots would indicate that there are two different orbitals with only one electron in them.

    Which way? You can draw them the way you did if you want to, and you don't care about other people being able to understand you, but the configuration is not as useful and it is not correct according to established conventions. Best to stick to the conventions.

    There is no 100% accurate notation, it's a notation. It's just a way of keeping notes about what is important to you. When other things are important, you use a different notation. It's standardized because other people want to be able to read your notes.

    As you advance in your studies you will probably get to learn more about the quantum mechanics behind these conventions and notations, and it will make more sense.
     
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