which is more Polar?

Discussion in 'DAT Discussions' started by Dencology, Jul 14, 2008.

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1. Dencology 2+ Year Member

396
0
Mar 7, 2008
Hey guys, i wanted to know if we have BCl3 and H2O and AlCl3 and PCl3, isn't it that all of them are polar. i mean if we draw BCl3, AlCl3, and PCl3 like the Mercedes Benz sign, (except for water) then we would have a dipole moment and it would be polar. Can anyone help to tell me if they are polar or not polar.

3. Glycogen 2+ Year Member

168
0
Jan 24, 2008
I think PCl3 is not polar b/c it has lone pair on top of P and so it will have symmetry,and dipoles will cancel each other out.

4. klutzy1987StudyingSucks Letsgo Mets 5+ Year Member

Jan 28, 2008
New York
BCl3 and AlCl3 are not polar and there molecular geometry is trigonal planar-120. H2O is obviously polar with its geometry being bent and bond angle of about 104. PCl3 is also polar because it has a lone pair on the P. Its bond angle is 107 and its molecular geometry is trigonal pyramidal.

5. klutzy1987StudyingSucks Letsgo Mets 5+ Year Member

Jan 28, 2008
New York
The lone pair will make it polar. Now there is a dipole as you said and therefore is polar. Nothing balances the lones pair therefore it is not symetrical.

6. Glycogen 2+ Year Member

168
0
Jan 24, 2008
How possibly can BCl3 and AlCl3 not being polar?
They only have 3 bonding which 2 of them will cancle one another and it will still have another bond which make it polar?
Am I missing something!

7. klutzy1987StudyingSucks Letsgo Mets 5+ Year Member

Jan 28, 2008
New York
Think of it like this. If you have tension fores in physics at three corners all separated by the same amount of degrees and they are pulling with the same force th overall force will be zero. Because the bonds are lined up so that the individual pull will overall cancel out the molecule is non polar. Remember that the substituents are separated by the same amount of degrees and are pulling (or pushing) with the same force. Therefore the sum of all vectors is zero and the sum of all polarity would be zero.

8. osimsDDS 5+ Year Member

925
1
Jun 8, 2007
SO/NOR-CAL
BCl3 and AlCl3 are trigonal planar, like dencology said they have the mercedes benz or the middle of a peace sign...so basically you have polor bonds from B to Cl on both sides going from B down to the right and from B down to the left. so that polar sign is ultimately down, but then you have a Cl on top of the B which has a polar bond facing up so that cancels the down polar sign which will give you a non-polar molecule...

9. Glycogen 2+ Year Member

168
0
Jan 24, 2008
So polarity is related to geometry.Can we assume that all the trigonal planars are nonpolar then.I don/t get it.

10. klutzy1987StudyingSucks Letsgo Mets 5+ Year Member

Jan 28, 2008
New York
If all substituents are the same then all trigonal planars are nonpolar i think. But go by an individual casen basis. There is no reason to generalize about all trigonal planars. Look at all the substituents and see if all charges and pulls balance. As a sorta general rule though if all substituents are the same and there are no lone pairs on the central atom then the molecule will be nonpolar.