GC question polarity

[UnimaasMED]

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whats the best way to identify polar molecules?

For example: which of the following molecules are polar?
CBR4, SO3, XeF4, ClF5, PBr5

 

[Dr NAT]

Best way is to draw them out. The first three are non-polar for sure, since they have the same elements cancelling each other out. Then, ClF5, both have charges of -1, which i think may cancel eath other out, but im not sure. Then PBr5, its shape it pyramidal and they have different charges, so it is polar.
Hope that helps. Good luck.

 

[gigawatt]

CBr4: (4,0) tetrahedral = nonpolar
SO3: (3,0) trigonal planar = nonpolar
XeF4: (4,2) square planar = nonpolar
ClF5: (5,1) sqaure pyramidal = polar
PBr5: (5,0) trigonal bipyramidal = nonpolar

(bonding domains, lone pairs) on central atom

Most of the time you can have a hunch based on number of substituents and symmetry, but I always make the Lewis structure to be sure. (Assuming I'll have time to do so on the DAT...most people say you're not pressed for time on the science section.)

 

[UnimaasMED]

CBr4: (4,0) tetrahedral = nonpolar
SO3: (3,0) trigonal planar = nonpolar
XeF4: (4,2) square planar = nonpolar
ClF5: (5,1) sqaure pyramidal = polar
PBr5: (5,0) trigonal bipyramidal = nonpolar

(bonding domains, lone pairs) on central atom

Most of the time you can have a hunch based on number of substituents and symmetry, but I always make the Lewis structure to be sure. (Assuming I'll have time to do so on the DAT...most people say you're not pressed for time on the science section.)

How do you know which shapes are polar though? Do you go with the angle? Or is that mentioned somewhere?

 

[duodenum]

you have to see whether there is a dipole moment. The only one with a dipole is ClF5 because it holds a lone pair on the chlorine atom

 

[gigawatt]

How do you know which shapes are polar though? Do you go with the angle? Or is that mentioned somewhere?

Well, in each of these cases, all the bonds on the central atom are identical and so the magnitude of each dipole will be identical. All you have to do is worry about direction.

Obviously, bonds 180 degrees apart will have dipoles that cancel. Also, anytime you have a plane that contains multiple bonds evenly distributed (trigonal planar, square planar), those dipoles will cancel, as well.

Take trigonal bipyramidal geometry as an example. The bonds pointing up and down cancel out. The remaining three bonds are evenly distributed within a plane so they cancel.