AS Chemisty Question
"In NH3 the bonds are polar and the molecule is non-symmetrical. Therefore there is an overall dipole.
In BF3, the dipoles cancel as the molecule has a symmetrical shape."
This is in the mark scheme as the answer for one of the questions. I understand all about polar molecules as we have studied them in class but we never mentioned symmetry.
The mark scheme really has confused me as it says NH3 is non-symmetrical, but surely it is, along the plane of any one of the N-H bonds ? So it's symmetrical, along with BF3, so what's symmetry got to do with ANYTHING!?
Thank you in advance for your help
EDIT!!!!! : i thank you for your answers so far but you have failed to realise, I UNDERSTNAD ENTIRELY about polarity and why NH3 is polar, im asking about why the mark scheme talks about SYMMETRY
In BF3, the dipoles cancel as the molecule has a symmetrical shape."
This is in the mark scheme as the answer for one of the questions. I understand all about polar molecules as we have studied them in class but we never mentioned symmetry.
The mark scheme really has confused me as it says NH3 is non-symmetrical, but surely it is, along the plane of any one of the N-H bonds ? So it's symmetrical, along with BF3, so what's symmetry got to do with ANYTHING!?
Thank you in advance for your help
EDIT!!!!! : i thank you for your answers so far but you have failed to realise, I UNDERSTNAD ENTIRELY about polarity and why NH3 is polar, im asking about why the mark scheme talks about SYMMETRY
(edited 13 years ago)
I was a bit confused with a similar question, so sorry if I'm wrong.
But in NH3 the central Nitrogen atom has a lone pair of electrons so that there is more repulsion between the bonded pairs and lone pairs of electrons, so there is a smaller bond angle between the bonded pairs, so it's arranged in a trigonal pyramidal, which isn't actually symmetrical.
But in NH3 the central Nitrogen atom has a lone pair of electrons so that there is more repulsion between the bonded pairs and lone pairs of electrons, so there is a smaller bond angle between the bonded pairs, so it's arranged in a trigonal pyramidal, which isn't actually symmetrical.
For NH3 N is the central atom and for BF3 B is the central atom. SO both central atoms have the same amount atoms sorrounding them but one is symmetrical and one isn't does this mean that you need to calculate bond angles.
Original post by oHellno
I was a bit confused with a similar question, so sorry if I'm wrong.
But in NH3 the central Nitrogen atom has a lone pair of electrons so that there is more repulsion between the bonded pairs and lone pairs of electrons, so there is a smaller bond angle between the bonded pairs, so it's arranged in a trigonal pyramidal, which isn't actually symmetrical.
But in NH3 the central Nitrogen atom has a lone pair of electrons so that there is more repulsion between the bonded pairs and lone pairs of electrons, so there is a smaller bond angle between the bonded pairs, so it's arranged in a trigonal pyramidal, which isn't actually symmetrical.
this still doesnt help me coz the pyramidal shape seems symetircal to me
ignore all the little dashes in this and pretend were looking down from where the lone pair is and pretend the angles are all 107 degrees for now:
---|
H-----H
--\--/
---N
---|
---H
---|
surely there is indeed symmetry through the lines i drew above and below the crudely drawn NH3 molecule - and the same would apply if I could draw this imaginary line going through either of the other 2 N-H bonds as each of the angles is 107 degrees???
confused much? is this symmetrical in the way i have described? if not, why not?
Original post by oHellno
it's quite hard to explain, but as nitrogen is more elctronegative, it's negative end is exposed because of the lone pairs. so overall the molecule has bone positive and negative dipoles. In a way, it's similar to why water is polar.
it's quite hard to explain, but as nitrogen is more elctronegative, it's negative end is exposed because of the lone pairs. so overall the molecule has bone positive and negative dipoles. In a way, it's similar to why water is polar.
bone?
Original post by oHellno
it's quite hard to explain, but as nitrogen is more elctronegative, it's negative end is exposed because of the lone pairs. so overall the molecule has bone positive and negative dipoles. In a way, it's similar to why water is polar.
it's quite hard to explain, but as nitrogen is more elctronegative, it's negative end is exposed because of the lone pairs. so overall the molecule has bone positive and negative dipoles. In a way, it's similar to why water is polar.
i understand that entirely, dont get me wrong
its just that in every mark scheme iv looked at for past papers for this specification, its talked about lack of symmetry in NH3, yet i see symmetry in this molecule and am wondering why it would mention something like that when it seems wrong and / or irrelevant ?!?!?!
do u get what im getting at now? i understand polarity, just not the symmetry comment in the mark scheme!!
You're meant to know about the effect that double bonds have on the shape of a molecule. The double bond pushes away the bonds with the hydrogen.
Now consider the centres of charge in your mind. One, where there is a double bond (highly negative) seperate from three positive hydrogens, and one when the bonds are evenly spaced out.
In BF3, the molecules are symetrical. What it means by this, is that they have the same angle between their bonds, and the importance of this is that the centre of positive and negative charge will be in the centre. The electronegative flourines, although apart from the nucleus, have their centre of charge in the nucleus (the centre of the molecule) because each of them has 120degrees between them.
In NH3, the positive hydrogens are pushed away by the highly negative double bond. As a result their centre of positive charge has now been moved away from the centre of negative charge.
As soon a the centres of charge are no longer on the same point, the molecule becomes a dipole.
Now consider the centres of charge in your mind. One, where there is a double bond (highly negative) seperate from three positive hydrogens, and one when the bonds are evenly spaced out.
In BF3, the molecules are symetrical. What it means by this, is that they have the same angle between their bonds, and the importance of this is that the centre of positive and negative charge will be in the centre. The electronegative flourines, although apart from the nucleus, have their centre of charge in the nucleus (the centre of the molecule) because each of them has 120degrees between them.
In NH3, the positive hydrogens are pushed away by the highly negative double bond. As a result their centre of positive charge has now been moved away from the centre of negative charge.
As soon a the centres of charge are no longer on the same point, the molecule becomes a dipole.
Original post by The Smeezington
bone?
Honestly have no idea what I was even trying to say now
Original post by conzag123
i understand that entirely, dont get me wrong
its just that in every mark scheme iv looked at for past papers for this specification, its talked about lack of symmetry in NH3, yet i see symmetry in this molecule and am wondering why it would mention something like that when it seems wrong and / or irrelevant ?!?!?!
do u get what im getting at now? i understand polarity, just not the symmetry comment in the mark scheme!!
its just that in every mark scheme iv looked at for past papers for this specification, its talked about lack of symmetry in NH3, yet i see symmetry in this molecule and am wondering why it would mention something like that when it seems wrong and / or irrelevant ?!?!?!
do u get what im getting at now? i understand polarity, just not the symmetry comment in the mark scheme!!
So the only problem is in seeing the lack of symmetry? If you put all the d+s by the hydrogens and d-s by the nitrogen would that not show that the spread of charges isn't symmetrical? I see what you're saying, but don't know how to explain the lack of symmetry
Maybe it will come to me when I'm not so tired lolTrigonal pyramidal
(edited 13 years ago)
Original post by conzag123
"In NH3 the bonds are polar and the molecule is non-symmetrical. Therefore there is an overall dipole.
In BF3, the dipoles cancel as the molecule has a symmetrical shape."
This is in the mark scheme as the answer for one of the questions. I understand all about polar molecules as we have studied them in class but we never mentioned symmetry.
The mark scheme really has confused me as it says NH3 is non-symmetrical, but surely it is, along the plane of any one of the N-H bonds ? So it's symmetrical, along with BF3, so what's symmetry got to do with ANYTHING!?
Thank you in advance for your help
EDIT!!!!! : i thank you for your answers so far but you have failed to realise, I UNDERSTNAD ENTIRELY about polarity and why NH3 is polar, im asking about why the mark scheme talks about SYMMETRY
In BF3, the dipoles cancel as the molecule has a symmetrical shape."
This is in the mark scheme as the answer for one of the questions. I understand all about polar molecules as we have studied them in class but we never mentioned symmetry.
The mark scheme really has confused me as it says NH3 is non-symmetrical, but surely it is, along the plane of any one of the N-H bonds ? So it's symmetrical, along with BF3, so what's symmetry got to do with ANYTHING!?
Thank you in advance for your help
EDIT!!!!! : i thank you for your answers so far but you have failed to realise, I UNDERSTNAD ENTIRELY about polarity and why NH3 is polar, im asking about why the mark scheme talks about SYMMETRY
If a molecule is symmetrical then the dipoles all act in opposite directions, cancelling the polar effect... so I understood from class anyway.
So even though it contains polar bonds, the overall molecule isn't polar.NH3 isn't symmetrical because of the lone pair, so it's polar.
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