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  1. #1
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    Default Lewis structure of XeF2

    When draw out the structure of XeF2, there are two bonds in Xe and e long paris around Xe, I'm really confused about it. If ONE bond count to TWO electrons, the there are 10 electrons around the Xe. But why the formal charge is still ZERO??




    Estimate the ONO bond angle in NO2? The answer it over 120 degree due to the radical. Could you please also explain it?


    Thank you!!
    Last edited by wangruoxi; 08-17-2011 at 06:10 PM.

  2. #2

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    The tricky part for your first question is the difference in how we count electrons for the purpose of the octet rule vs for determining formal charge. For the purpose of the octet rule we count all shared and unshared electrons and in XeF2 xenon does indeed have 10 electrons around it and exceeds the octet rule (remember that it's alowwed to as it's in the 3rd row or lower). But for formal charge we only count half of the shared electrons (the bonding electrons) as belonging to either atom and so for determining the formal charge for xenon we would say that it has 6 unshared electrons that are his alone and then half of the bonding electrons (2 bonds so 2 more additional electrons) for a total of 8 valence electrons for the purpose of determining formal charge. As xenon's normal number of valence electrons is 8 it ends up with no formal charge.

    For the bond angles in NO2, this is definitely tricky and an exception to the normal rules. I've attached an image below of the lewis structure for NO2.
    Click image for larger version. 

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    Note that the radical electron on the nitrogen isn't what we would normally consider a complete electron domain (usually a lone pair of electrons). I therefore doesn't result in as much of a repulsion away from the bonds. If the repulsions between all 3 'domains' were equal the O-N-O bond angle would be exactly 120 degrees. But since the bonds are repelled by each other more than by the radical electron they will end up being more than 120 degrees apart. A very tricky example.

    Hope this helps!

  3. #3
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    Thanks a lot, Chad!

    Could you please give some example molecules which I need to consider the radical in DAT?

  4. #4

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    The much more common radical you'll see would be like a methyl radical. It's only sp2 hybridized (the lone electron is in an unhybridized p orbital) and the bond angles are exactly 120 degrees.

    Hope this helps!

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