Why is boron trichloride non-polar but phosphorous trichloride polar?

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emilyjeh
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I've got some chemistry homework to do and it's all to do with bonding and structure.

The only one I can't think of an explanation for is this question: "Boron trichloride is non-polar but phosphorous trichloride is polar. Explain."

I get that it will probably have something to do with the Pauling scale and electronegativities but I can't figure it out.

Any help would be great!
Thanks
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charco
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(Original post by emilyjeh)
I've got some chemistry homework to do and it's all to do with bonding and structure.

The only one I can't think of an explanation for is this question: "Boron trichloride is non-polar but phosphorous trichloride is polar. Explain."

I get that it will probably have something to do with the Pauling scale and electronegativities but I can't figure it out.

Any help would be great!
Thanks
It has more to do with shape and symmetry.

Remember that a difference in electronegativity causes bonds to be polar and that a dipole is a vector quantity with both magnitude and direction.

Opposite dipoles cancel out and parallel dipoles add up.

Dipoles can be resolved into horizontal and vertical vectors using the parallelogram rule.

Look at the shape of the two molecules and see if you can apply the above ideas.
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emilyjeh
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(Original post by charco;[url="tel:60610031")
60610031[/url]]It has more to do with shape and symmetry.

Remember that a difference in electronegativity causes bonds to be polar and that a dipole is a vector quantity with both magnitude and direction.

Opposite dipoles cancel out and parallel dipoles add up.

Dipoles can be resolved into horizontal and vertical vectors using the parallelogram rule.

Look at the shape of the two molecules and see if you can apply the above ideas.
Ah ok, I'll look at their shapes and see how that will affect it (we haven't looked at shapes of molecules yet but I'll read into it)

Thank you!
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therecovery
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(Original post by emilyjeh)
I've got some chemistry homework to do and it's all to do with bonding and structure.

The only one I can't think of an explanation for is this question: "Boron trichloride is non-polar but phosphorous trichloride is polar. Explain."

I get that it will probably have something to do with the Pauling scale and electronegativities but I can't figure it out.

Any help would be great!
Thanks
Boron trichloride has no lone pairs within its bonding but phosphorous trichloride has within its phosphorous atom.
Due to the presence of the lone pair, the positive charges on the molecule wont exactly coincide with the negative charges. Due to the uneven charge cancellation, there will be a resultant net dipole. When there is a resultant net dipole, there will be dipole dipole attractions which will cause a molecule to become polar.

I think shape of symmetry arise because of the polar bonds (dipole dipole interactions). The symmetry/shape does not cause the molecule to have a charge but rather the bonding within it influences its overall charge.

I figured this out by using NH3. N is in the same group as phosphorous. hence, it has the same number of electrons in the outer shell and therefore similar chemical properties which include bonding. in addition, NH3 has an overall charge of +1 which makes it polar and quite similar in terms of polarity to phosphorous trichloride.
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KombatWombat
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I think you need to be a bit more careful with your terminology!
(Original post by therecovery)
Boron trichloride has no lone pairs within its bonding but phosphorous trichloride has within its phosphorous atom.
Boron has no lone pairs, but each chlorine has 3, so you can't say boron trichloride has no lone pairs! Lone pairs pretty much mean non bonding so having no lone pairs 'within its bonding' doesn't mean a whole lot.

Due to the presence of the lone pair, the positive charges on the molecule wont exactly coincide with the negative charges. Due to the uneven charge cancellation, there will be a resultant net dipole. When there is a resultant net dipole, there will be dipole dipole attractions which will cause a molecule to become polar.
In BCl3 it's not really the partial positive charges cancelling with the partial negative charges, but the negative charges cancelling each other out by pulling in opposing directions. It's like a tug of war with both sides pulling exactly as hard.

I think shape of symmetry arise because of the polar bonds (dipole dipole interactions). The symmetry/shape does not cause the molecule to have a charge but rather the bonding within it influences its overall charge.

I figured this out by using NH3. N is in the same group as phosphorous. hence, it has the same number of electrons in the outer shell and therefore similar chemical properties which include bonding. in addition, NH3 has an overall charge of +1 which makes it polar and quite similar in terms of polarity to phosphorous trichloride.
Dipole moments aren't overall charges! NH3 is a neutral molecule, but has a dipole moment because there's more charge on the nitrogen. You can work out the shapes with VSEPR.
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therecovery
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(Original post by KombatWombat)
I think you need to be a bit more careful with your terminology!

Boron has no lone pairs, but each chlorine has 3, so you can't say boron trichloride has no lone pairs! Lone pairs pretty much mean non bonding so having no lone pairs 'within its bonding' doesn't mean a whole lot.

In BCl3 it's not really the partial positive charges cancelling with the partial negative charges, but the negative charges cancelling each other out by pulling in opposing directions. It's like a tug of war with both sides pulling exactly as hard.



Dipole moments aren't overall charges! NH3 is a neutral molecule, but has a dipole moment because there's more charge on the nitrogen. You can work out the shapes with VSEPR.
Thank you so much, Yes, you are right about the boron lone pairs and it was wrong of me to admit that BCl3 does not have any lone pairs at all.

https://en.wikipedia.org/wiki/Ammonia

"The nitrogen atom in the molecule has a lone electron pair, which makes ammonia a base, a proton acceptor. This shape gives the molecule a dipole moment and makes it polar."
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therecovery
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and yes thank you pointing out that dipole moments arent the overall charge. indeed dipole moments only affect the polarity. however, nh3 has a charge of +1. how is this charge possible?
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KombatWombat
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(Original post by therecovery)
and yes thank you pointing out that dipole moments arent the overall charge. indeed dipole moments only affect the polarity. however, nh3 has a charge of +1. how is this charge possible?
You're misreading the sentence you quoted. Ammonia absolutely is a neutral molecule!

The nitrogen atom in the molecule has a lone electron pair, which makes ammonia a base, a proton acceptor
This means NH3 can accept an extra proton, making it a base:
NH3 + H+ ⇌ NH4+

NH4+ has an extra proton, but no extra electrons, so it does have a charge of +1. NH4+ is called 'ammonium' - maybe the similar names is where you're getting confused?

Draw a lewis structure of NH3, and count the number of electrons vs protons. You'll get the same number!
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