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Thread: Question about polarity

  1. #1 Question about polarity 
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    Can someone explain why ozone is a polar molecule? I thought that polarity was based on the differences in electronegativity between atoms and the geometry of the atom. I thought ozone would be a nonpolar molecule, since despite the bent shape, all of the atoms are oxygen, so they would be pulling the electrons equally. Apparently this is not true, so why is it polar, and where would the dipole moment be pointing? Thanks alot in advance.


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  3. #2  
    Moderator Moderator AlexP's Avatar
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    The only way three oxygen atoms can bond in a linear fashion such as in ozone is for the molecule to adopt resonance structures. One structure has the double bond between two of the oxygen atoms, with the remaining oxygen only having a single bond. The other structure is the same, but with the double bond between the other two oxygens. In either structure, the middle oxygen will have a positive charge because it is forming three bonds, and the single-bonded oxygen will have a negative charge. In reality though, you treat it like ozone has both structures at the same time. Instead of a double bond and a single bond ozone has two "1.5 bonds." So it has charges, making it polar, because by necessity it adopts these resonance structures. It's the only way three oxygen atoms can bond in that way. They can't all achieve the two bonds they need and not have a formal charge.

    Hopefully I explained that ok. Make sense? Look it up on Wikipedia for a picture of the structure.


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    Reptile Dysfunction drowsy turtle's Avatar
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    Presumably, being bonded to an oxygen atom on either side pulls the electrons away from the centre atom.
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    Hi, thanks for your responses but I'm still a bit confused... I understand the part about forming resonance structures, but I got lost when it comes to formal charge.

    Quote Originally Posted by Chemboy
    So it has charges, making it polar, because by necessity it adopts these resonance structures. It's the only way three oxygen atoms can bond in that way. They can't all achieve the two bonds they need and not have a formal charge.

    Hopefully I explained that ok. Make sense? Look it up on Wikipedia for a picture of the structure.
    So in addition to electronegativity and shape, the formal charge will also affect the polarity of the molecule?

    Quote Originally Posted by drowsy turtle
    Presumably, being bonded to an oxygen atom on either side pulls the electrons away from the centre atom.
    I'm confused on why it happens, since won't two oxygens pull the electrons with equal strength, making no dipole? So for example, H2O is polar, since the Oxygen atom pulls electrons more than the Hydrogen atoms. But in ozone, they are all oxygen atoms so why the bottom side is more negative?
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    Moderator Moderator AlexP's Avatar
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    The best way I'm able to explain it is that because all the oxygens can't form two bonds, ozone forms resonance structures. Due to the bonding, not the electronegativity, there are two negative charges and one positive charge, so there is a net charge which gives rise to a polar molecule.
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    OK, I think I understand it better. Thanks everyone for your help.
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