how do you know if a molecule is polar?

Question

please help!!

Answer 1

Be very careful:
First, look at the electronegativity differences, the presence of groups like OH- or cations (like Na+ of soap)
Then, you must know molecular geometries, it is highly important:
For BF3 this is a trigonal planar compound, all differences of electronegativity B [SnCl3]- this is a trigonal pyramid compound.
Cl
I
:Sn-Cl
I
Cl
AS you can see, Sn has a lone pair of electrons, making Cl atoms accommodate in the base corners of an imaginary pyramid. Even if Cl cancel each others dipolar momentum on the sides, they all point ↓. This is a polar compound.

Answer 2

There's an easy practical test for a polar liquid. Take water. Let it drip from a tap. On a dry day, run your comb through your hair a few times to give it an electrostatic charge. Then hold it close to the falling drops. Instead of falling vertically, they'll be deflected. The end of the water molecule with the opposite charge to the comb is attracted towards the comb. Being closer, its attraction to the comb is greater than the repulsion of the other end. Now try it with a non-polar liquid like petrol. No deflection takes place. I've never tried this with a polar solid like grains of salt, but it should work if the grains are orderly crystals, not amorphic.

Answer 3

It is polar if it has a slightly positive charge on one side and slightly negative on the other. So first draw a structure diagram, then look at electronegativities to see if one atom pulls the electrons closer, making that side negative.

Answer 4

If it acts all nice and then suddenly becomes angry.