least polar to most polar?

Question

i need to rank the following in order from least polar to most polar, i do not know what i need to look at? molecular shape? help pleez!
CH2Br2, CF2Cl2, CH2F2, CH2Cl2, CBr4, CF2Br2

Answer 1

First you need to look at the difference in the electronegativities of the atoms in each bond. That will determine the dipole moment of each bond. Then you need to look at the geometry to see what the sum of the individual molecules will be (if they cancel out each other)

All of your molecules have tetrahedral geometry
The electronegativity series for the elements you have is

F>Cl>Br>C>H

So all C-X bonds where X is F,Cl,Br have a partial charge on X and a positive on C, while the C-H bond has the opposite polarity.

CB4 is not polar because it has four identical polar bonds which cancel out each other. The other molecules have not all four bonds identical, therefore they will have a net dipole moment.

F and Cl are the closest in terms of electronegativity. Thus the individual dipole moments will almost cancel out each other giving a very small net dipole moment (think of it as CF4 with small changes in the 2 F atoms)

F and Br have a bigger electronegativity difference than F and Cl, so the CF2Br2 will have higher dipole moment than CF2Cl2

In CH2X2 the C-H dipole moments have the reverse polarity than C-X. So instead of counteracting the dipole moments of the C-X bonds, they are strengthening them. So for the CH2X2 molecules the dipole moment will increase as the C-X electronegativity difference increases.

To summarise, the order is

CBr4 < CF2Cl2

Answer 2

Least Polar

Answer 3

I would try to look at strong asymmetries in the molecules, those would be the most polar.

If my memory doesn't fail there was something called electronegativity

http://en.wikipedia.org/wiki/Electronegativity

There you can find a table, and see that F is more electronegative than Cl, which is more electronegative than Br, which is more electronegative than H.

So the most asymmetrical molecule with F will be the most polar:
CH2F2
The most symmetrical molecule, CBr4 would be the least polar.

Basically I would say that you have to look at the difference of electronegativity of the atoms on opposite sides of the molecules. The most electronegative atoms will make the bonding electrons to be closer to them, resulting in asymmetric electrical charge distribution along the molecule, and thus, making the molecule polar.