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What makes a Molecule Polar? Dipole?

  1. Dec 6, 2009 #1
    Hi,
    1st year chem guy here...I'm missing this idea. Is it all based upon Electronegativity?

    I can see why HBr or HCl and other 2 atom compounds would be polar. 2 atoms with different charges or even sizes creating an un-even pull.

    I'm guessing H2O is polar as the 4 remaining electrons on the oxygen molecule create an imbalance.

    I'm guessing CO2 is non polar because the O=C=O balances perfectly negating/balancing the charges....

    But if you throw CCl4 at me I can find a definition that it is non-polar but couldn't show the math etc to explain it....

    Our textbook explains it thusly:
    "the greater the electronegativity difference, the more polar." And I understand the Pauling scale how differences determine either ionic or covalent

    But HBr for instance has an electronegativity difference of 0.7 and is polar, yet CO2 difference is 0.5 and is non polar. That's pretty close. Wait a sec....is it because the CO2 molecule is balanced with one O on either side? Would CO be considered polar as it is just 2 atoms?

    Could I assume ALL 2 atom compounds (ionic or molecular) are polar!?

    Thanks,

    Warren
     
  2. jcsd
  3. Dec 6, 2009 #2
    What is the shape of the CCl4 molecule?
     
  4. Dec 6, 2009 #3
    Like you said, it's based on electronegativity, but shape plays a huge role in polarity. You were right, CO2 is non polar because of the symmetrical shape of the molecule. The dipoles cancel each other out. CO, on the other hand, would be polar because there isn't a second bond to cancel out the first. All bonds between atoms of different electronegativities are slightly polar. Non polar usually only means that the polarity is small enough to be ignored.

    Here's the problem with your proposed rule. When you have a diatomic gas (O2, N2, etc.), the two atoms have the same electronegativity, so the elecrons are shared evenly, making it non polar. In this case, non polar really does mean absolutely non polar. Does this make sense?
     
  5. Dec 6, 2009 #4
    Tetrahedal.

    So is one shape more apt to be polar or enhance any electromagnetic differences?

    Would one expect linear molecules to be polar? Except in cases of H2, O2, N2 etc...

    Warren
     
  6. Dec 6, 2009 #5
    Thanks,

    It makes sense...For some reason it's not explained so simply in our text book. It's all mixed in with a discussion of electronegativity but the "rules" aren't really touched upon. I thought I had it before.....

    I'm working some excersizes now and if I find a weird one I will post.

    Thanks,

    Warren
     
    Last edited: Dec 6, 2009
  7. Dec 7, 2009 #6

    It's partly do with symmetry arguments, the more highly symmetrical a structure is, the less polar it is.

    So tetrahedral structures, like carbon tet, or square planar species, like NiCl42- are less polar, than say, SF4
     
  8. Dec 7, 2009 #7
    Get a molecular model set and make a tetrahedral molecule (ex: CCl4). The bonds are all equally spaced and the 4 attached atoms all have the same electronegativity. Cl is more electronegative than C, but remember that they're all equally spaced which makes the molecular non-polar.
     
  9. Dec 7, 2009 #8
    Some molecules have polar bonds but that does not necessarily mean that the molecule is polar. The polarity of the molecule depends on the net pull of electrons (electronegativity) within the molecule. If the electronegativities of the peripheral bonds are equal and the bond lengths between the atoms are equal, the chances are it's non-polar. However, if there is an unequal distribution of electronegativity within the molecule the molecule will be polar. The more uneven the distribution, the more polar the molecule.
     
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