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    Hi I just wondering if someone could briefly run through polarity; dipoles and polar molecules. As I'm revising for my Exam that is in 5 days and I'm a little clueless and confused
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    Hi just wondering if someone could briefly run through polarity;dipoles and polar molecules. As I am revising for my exam that is in 5 days and I'm a little clueless and confused on this topic Please Help
    (PS posted this thread before in wrong forum, sorry for replica)
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    (Original post by Josh989)
    Hi just wondering if someone could briefly run through polarity;dipoles and polar molecules. As I am revising for my exam that is in 5 days and I'm a little clueless and confused on this topic Please Help
    (PS posted this thread before in wrong forum, sorry for replica)
    Molecules are held together by covalent bonds that may be considered as a region of electron (negative) density between positive nuclear centres.

    It is observed that some atoms attract electrons towareds themselves along the bond more than others. Such atoms are said to be electronegative.

    By comparison of all atoms with one another an electronegativity scale (actually calculated from a consideration of the ionisation energy nad the electron affinity) showing the relative electronegativity of all of the atoms can be built up.

    Bond polarity is caused by atoms with different electronegativity being bonded together. The more electronegative atom attracts more of the electron density of the bond than the other. This creates a non-homogeneous electrostatic field in which the more electronegative atom has a slightly more negative charge than the less electronegative atom.

    This bond is then said to have a dipole.

    The dipole is characterised by its magnitude and its separation, i.e. it is a vector quantity.

    When a molecule has more than one dipole they can be resolved like any other vector quantity to give an overall dipole. If there is an overall dipole then the molecule is said to be polar.

    How you doing so far?
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    Ahh ok, but how would this be answered... Water and carbon dioxide both have polar bonds. Explain why water has polar molecules but carbon dioxide has non-polar molecules? (past paper question) How do you distinguish between polar and non-polar?
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    Ahh ok, but how would this be answered... Water and carbon dioxide both have polar bonds. Explain why water has polar molecules but carbon dioxide has non-polar molecules? (past paper question) How do you distinguish between polar and non-polar?
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    (Original post by Josh989)
    Ahh ok, but how would this be answered... Water and carbon dioxide both have polar bonds. Explain why water has polar molecules but carbon dioxide has non-polar molecules? (past paper question) How do you distinguish between polar and non-polar?
    If you read what I wrote ...

    The individual dipoles can all be resolved into horizontal and vertical components (and 'z' axis if needed).

    If they all cancel out (i.e. are equal and opposite) then the molecule is non-polar if not then the molecule is polar.
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    Do you know about temporary dipoles too?

    Van der Waals forces occur due to temporary dipoles. Electrons in an atom are constantly moving which creates instantaneous dipoles. These instantaneous dipoles induce more dipoles in neighbouring atoms. The attraction between dipoles is the Van der Waals force
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    (Original post by v1k2a3t4)
    Do you know about temporary dipoles too?

    Van der Waals forces occur due to temporary dipoles. Electrons in an atom are constantly moving which creates instantaneous dipoles. These instantaneous dipoles induce more dipoles in neighbouring atoms. The attraction between dipoles is the Van der Waals force
    Van der Waals forces are the collective name for all intermolecular bonds :P

    But yes, http://www.chemguide.co.uk/atoms/bonding/vdw.html explains everything you will ever need to know.
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    Ahh cheers guys it all makes much more sense now... Also if looking whether a molecule is polar/non-polar it mainly depends on whether or not the molecule is symmetrical e.g CO2 or asymmetrical e.g H2O = CO2 being the non-polar one & H2O being polar, doesn't it?
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    (Original post by Josh989)
    Ahh cheers guys it all makes much more sense now... Also if looking whether a molecule is polar/non-polar it mainly depends on whether or not the molecule is symmetrical e.g CO2 or asymmetrical e.g H2O = CO2 being the non-polar one & H2O being polar, doesn't it?
    Yes, symmetry is very important if it means that the individual dipoles cancel out, as they do with carbon dioxide.

    Name:  CO2_polarity.gif
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    The vertical component of the individual dipoles do not cancel out with water:

    Name:  water_polarity.gif
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    Thanks for everyone's help, I think I understand it now!!!! (or at least the main basics)
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    (Original post by Another)
    Van der Waals forces are the collective name for all intermolecular bonds :P

    But yes, http://www.chemguide.co.uk/atoms/bonding/vdw.html explains everything you will ever need to know.
    Oops my bad! My chemistry teacher can be very confusing Will check out that website though!
 
 
 
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