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Difference between bond polarity and molecular polarity?

  1. Dec 24, 2004 #1
    what is the difference between bond polarity and molecular polarity?
     
  2. jcsd
  3. Dec 24, 2004 #2

    t!m

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    Take for example [tex]BF_3[/tex].

    Its structure is a central Boron atom single bonded to 3 Fluorine atoms. The molecule, as a whole, is not polar, because it is completely symmetrical, and thus one could not point to a more negative side. Each of those single bonds, on the other hand, is polar, because Fluorine is more electronegative than Boron.
     
  4. Dec 24, 2004 #3
    so does HCl have a polar or nonpolar "molecular bond"? I wrote down polar but I think it has to be nonpolar acording to what you said
     
    Last edited: Dec 24, 2004
  5. Dec 24, 2004 #4
    Since HCl molecule is defined as a molecule that consist of a hydrogen atom bonded to a chlorine atom, and the bond is polar, this also implies that the molecule is polar.
     
    Last edited: Dec 24, 2004
  6. Dec 24, 2004 #5
    how is the molecule polar if the structure is symmetrical?

    does this mean every polar bond must have be a polar molecule?
     
  7. Dec 24, 2004 #6
    For instance, for a bond to be polar, there must be an unequal distribution of electrons amongst the bonded pair, due to the unequal electronegativity. Usually the less electronegative element is in the center of the molecule. However, if the molecule is symmetrical, by vector-adding, the attraction of the electrons is "cancelled" out since a substance on the opposite end provides an equal attractive force , due to the symmetry, upon the electrons.

    We can use CO2 as an example

    O=C=O is a symmetrical molecule, but it consists of polar C=O bonds. However, due to the symmetry, the O on each end cancels out the unequal attractive forces on the electrons, and thus the molecule is nonpolar.
     
    Last edited: Dec 24, 2004
  8. Dec 24, 2004 #7
    all I know is that symmetrical molecules are nonpolar molecules. That means linear and tetrahedral bonds are considered nonpolar molecules. However, if the EN is between .5-1.7 then it has a polar BOND. So, back to HCl

    H-Cl has a linear bond, that means it is a nonpolar molecule.
    However, HCl is a EN difference over .5 which means it is a polar bond.

    Why is HCl a polar molecule as you said?
     
  9. Dec 24, 2004 #8
    because that's all the molecule is made of, just the linear polar bond of hydrogen and chlorine. There's nothing there to counter-act the electronegativity difference, unlike in a symmetrical molecule, and thus the molecule is polar. And by the way, being a linear molecule does not mean it will always be symmetrical, for instance HCl molecule. In general, if the opposite sides do not resemble each other, then the molecule should not be symmetrical (this is going on a limb here, but it works in most cases)...
     
  10. Dec 24, 2004 #9
    what about H2CO? it's a polar molecule and also has a polar bond right?
     
  11. Dec 24, 2004 #10
    You are correct.
     
  12. Dec 24, 2004 #11
    would this molecular structure be called bent or pyramidal? or is it called something else?
     
  13. Dec 24, 2004 #12
    Did your teacher explain to you about the different molecular geometries? I'll help you out. Well there's 0 nonbonding pairs and 3 bonding pairs... There's your hint...
     
  14. Dec 24, 2004 #13

    t!m

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    Oops, sorry if I spoiled that one. Answer in white.

    The structure of [tex]H_2CO[/tex] would be trigonal planar, with 120 degree bond angles.

    And in all fairness, apchemstudent, there are 0 nonbonding pairs and 4 bonding pairs, as the carbon double bonds to the oxygen. More accurately, the defining characteristic of the molecule is that there are 3 regions of electron density.
     
    Last edited: Dec 24, 2004
  15. Dec 24, 2004 #14
    great job... way to help someone learn...
     
  16. Dec 24, 2004 #15
    HCN has a linear bond, so that means that it is a nonpolar molecule. However, how did I know if the bond is polar or not? since H-C is polar and C=N is nonpolar, what would be the bond polarity?
     
  17. Dec 24, 2004 #16

    t!m

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    Linear bonds are not always nonpolar. I think you're overgeneralizing the concept of symmetry. HCN is actually a polar molecule. Also, both bonds are polar as well, which is actually the reason for the molecule's polarity. Try this site.
     
  18. Dec 24, 2004 #17
    why is C=N a polar bond? It has an EN of .4, doesnt that make it nonpolar?
     
  19. Dec 24, 2004 #18

    t!m

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    Ack, this sort of thing really depends on teacher. No bond is truly nonpolar except for bonds between identical atoms, i.e. C-C bonds. If you're following the rules for END, then yes, I suppose, C=N is a nonpolar bond.
     
  20. Dec 28, 2004 #19
    I can not give you the kind of in-depth answer many here can... But try this:

    In high school I learnt there are three kinds of bounds: covalent bounds, polar covalent bounds and ionic bounds. Between some electronegativity (EN) values the bound is called covalent, in others, polar covalent or ionic. This classification is good to know because it roughly predicts solubility, reactivity and polarity - ionic bounds are polar and covalent bounds tend not to be polar.

    However, according the all mighty quantum mechanical model the orbitals (where the electrons reside) are in fact "electron probability densities" - at a higher density an electron is more likely to be found. Hence, according to this model, no electron swapping really takes place in an ionic bound, the density of electrons just becomes higher close to one of the bounding atoms. The electron density is in fact slightly higher close to the N atom than it is close to the C atom, because of the small, yet existing, difference in EN. That is probably why t!m said it was polar. (But isn't the CN bound actually a triple bound, in HCN? I think the lewis structure would suggest that...)

    When thinking of the molecular polarity of HCN, in addition to the bound polarities and molecular structures, there is one more nifty (but not necessary, i presume) thing to keep in mind when determining the magnitude of molecular polarity. Since the the EN values increase from H to C to N, the CN bounds should be slightly more polar in the HCN molecule than by it self. (And therefore the HC bound should be slightly less polar?) That is because the electron density of the HC bound is so close to the C atom that it will repel the electron density in the CN bound further towards the N atom.

    Now I hope I have not made a complete ass of myself with my first post and confused you, urbanxrisis even more. So, please ask what you've missed or correct whatever I've missed! :smile:
     
  21. Dec 28, 2004 #20
    Yes. C=N bond is nonpolar, but once you bond it with hydrogen ion, H+ and CN- forms together and becomes polar. Due to the positive and negative attraction in between these two ions, Hydrogen ion(the positive end of the polar vector) reaches, or becomes pulled towards the CN- ion(also known as the negative end of the polar vector which attracts the positive end).
    Also, this can be seen when you draw this molecule using vector analysis.

    -> = single bond(attracted to the right), ---> = triple bond(attracted to the right)

    H -> C --->N

    since hydrogen is attracted to carbon, we have a bond polarity going to the right. If this molecule was to be a nonpolar molecule, we need another vector line going inward towards the carbon from the same direction with the same length(length is determined by the attractiveness + bond type between the atoms). In our case, we have another vector line going to the right. Thus, does not cancel out the dipole moment, and making it have a permenant dipole moment. Hope this helps anybody. Correct me if im wrong anybody~ thanks. :smile:
     
    Last edited by a moderator: Dec 28, 2004
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