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Bond Polarity and Non Polar Molecules Question!

Right ok 1st of all i am sorry if i dont make much sense but i will try too :P LOL, right ok, I understnad that a permanent dipole arises due to the slight charge difference across a bond to due the difference in electronegativites of the bonded atoms. And i understand that a polar molecule has an overall dipole when you take into account all dipoles across the bonds. And i also get how non polar molecules can have polar bonds due to them being symmetrical and the dipoles acting in different directions cancelling each other out. I however do not seem to understand how to determine whether a molecule is symmetrical or not. for example why is BF3 non polar whilst PF3 is polar, i know BF3 is trigonal planar due to the 3 bonded regions and PF3 is Pyramidial due to the 4 bonded regions ie 3 electron pairs and 1 lone pair. but i dont see how PF3 is any different to BF3 in terms of symmetry and dipole acting directions. its the same case with h20 being polar whilst co2 is non polar, h20 2 bonded 2 lone so non liner.. co2 2 bonded so linear but once again i dont understand the symmetry and dipole acting directions behind it all :s-smilie:
Any help would be appreciated
thank you :smile:
- Grant

Thank you for your time
gmartin2k8
Right ok 1st of all i am sorry if i dont make much sense but i will try too :P LOL, right ok, I understnad that a permanent dipole arises due to the slight charge difference across a bond to due the difference in electronegativites of the bonded atoms. And i understand that a polar molecule has an overall dipole when you take into account all dipoles across the bonds. And i also get how non polar molecules can have polar bonds due to them being symmetrical and the dipoles acting in different directions cancelling each other out. I however do not seem to understand how to determine whether a molecule is symmetrical or not. for example why is BF3 non polar whilst PF3 is polar, i know BF3 is trigonal planar due to the 3 bonded regions and PF3 is Pyramidial due to the 4 bonded regions ie 3 electron pairs and 1 lone pair. but i dont see how PF3 is any different to BF3 in terms of symmetry and dipole acting directions. its the same case with h20 being polar whilst co2 is non polar, h20 2 bonded 2 lone so non liner.. co2 2 bonded so linear but once again i dont understand the symmetry and dipole acting directions behind it all :s-smilie:
Any help would be appreciated
thank you :smile:
- Grant

Thank you for your time


You can think of dipoles as vectors that can be resolved into vertical and horizontal components into two or three dimensions.

Water dipoles cancel out in the horizontal resolution but add up in the vertical. Hence water has an overall dipole.



Carbon dioxide dipoles cancel out in the horizontal and there are no vertical resolutions to consider.

The same treatment can be carried out in three dimensions for BF3 and PF3
Reply 2
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gmartin2k8
OP
charco
You can think of dipoles as vectors that can be resolved into vertical and horizontal components into two or three dimensions.

Water dipoles cancel out in the horizontal resolution but add up in the vertical. Hence water has an overall dipole.



Carbon dioxide dipoles cancel out in the horizontal and there are no vertical resolutions to consider.

The same treatment can be carried out in three dimensions for BF3 and PF3


Thank you :smile: That really helped tbh + rep for you sir :smile:
It is appreciated
So basically BF3 is non-polar because it has no lone pairs, its shape is symmetrical (triganol planar) and its dipoles cancel. PF3 is polar because it has one lone pair, it's asymmetrical (pyramidal) and its dipoles don't cancel?

As chem unit 1 OCR
Original post by motivatedshroom
So basically BF3 is non-polar because it has no lone pairs, its shape is symmetrical (triganol planar) and its dipoles cancel. PF3 is polar because it has one lone pair, it's asymmetrical (pyramidal) and its dipoles don't cancel?

As chem unit 1 OCR


Yes. The dipoles in PF3 cancel out in the horizontal plane, BUT do not cancel out in the vertical.

BF3 is planar and so has no resolved vector dipoles in the vertical direction.
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user1-4
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Original post by charco
You can think of dipoles as vectors that can be resolved into vertical and horizontal components into two or three dimensions.

Water dipoles cancel out in the horizontal resolution but add up in the vertical. Hence water has an overall dipole.



Carbon dioxide dipoles cancel out in the horizontal and there are no vertical resolutions to consider.

The same treatment can be carried out in three dimensions for BF3 and PF3


Sorry I don't understand where the two red arrows on the water diagram come from. Aren't there two lone pairs on top of the water? So why aren't there any arrows there? And why two at the bottom?

Thanks in advance.
Original post by user1-4
Sorry I don't understand where the two red arrows on the water diagram come from. Aren't there two lone pairs on top of the water? So why aren't there any arrows there? And why two at the bottom?

Thanks in advance.


A dipole is a vector quantity in that it has both magnitude and direction.

Each O-H bond is polarised in the direction of the bond. i.e. the magnitude is the size of the dipole and the direction is along the bond.

This dipole can be resolved mathematically using the parallelogram law into vector components in the horizontal and vertical directions.

The arrows represent the resolved dipole vectors in the horizontal and vertical directions.
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user1-4
1
Original post by charco
A dipole is a vector quantity in that it has both magnitude and direction.

Each O-H bond is polarised in the direction of the bond. i.e. the magnitude is the size of the dipole and the direction is along the bond.

This dipole can be resolved mathematically using the parallelogram law into vector components in the horizontal and vertical directions.

The arrows represent the resolved dipole vectors in the horizontal and vertical directions.


ok so i understand because the OH bond goes diagnolly the vectors is horizontal then vertical. Thus two red arrows downwards. Do the lone pairs have no effect on the polarisation?

Thanks again :colondollar:
Original post by user1-4
ok so i understand because the OH bond goes diagnolly the vectors is horizontal then vertical. Thus two red arrows downwards. Do the lone pairs have no effect on the polarisation?

Thanks again :colondollar:


No, the lone pairs only have an effect in that they are a characteristic of oxygen, which is an electronegative element.

The key to dipoles is the difference in electronegativity between the two atoms bonded together.

For example, in the bond between oxygen and bromine, the oxygen is more negative than bromine even though bromine has three lone pairs to oxygen's two.
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user1-4
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thanks
Reply 10
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user1-4
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sorry, but how would i draw that diagram that you did for water with a molecule such as CH4?
Original post by user1-4
sorry, but how would i draw that diagram that you did for water with a molecule such as CH4?


You would have to do a three-dimensional coordinate treatment. It is perfectly possible, but would be fairly tedious.

The easiest way would be to do a two dimensional treatment with one of the three lower dipoles and the upper vertical diplole and demonstrate that the vertical component of the lower dipole is 1/3 that of the upper dipole.

The tetrahedral bond angle is 109º 28', which means that the angle subtended between the bond and the vertical is 78º 32'.

Hence the resolved vector = x * cos 71º 32' = 0.33 x

Where x is the magnitude of each dipole.

Hence the three lower dipoles resolve to x in the opposite direction to the upper dipole.

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