[somegirl90]

I just started taking orgo 1 and it's be a while since I had chem 1 and 2 so I'm not sure if this is still just review and belongs in general chemistry but I figured I'd try here first. I know I probably learned this but I don't remember. I tried looking online but I'm still unsure how to tell if a molecule has a molecular dipole.

The question I'm trying to answer is: which of the following molecules has a molecular dipole?

A. H          H
      \       /
        C=C
      /       \
    H           H

B. F          F
      \       /
        C=C
      /       \
    H          H

C. H          F   Based on what I understand of what I read online I believe it's this one because the bond
      \       /     dipole moments don't cancel out but I'm not sure.
        C=C
      /       \
    F          H

D. F          F
      \       /
        C=C
      /       \
    F          F
Any help and or explanation to help me understand would be greatly appreciated. I have a few more of these to do but I can probably figure it out if I can understand this.

[synthon]

In C, the F atoms pull electron density away from the double bond in opposite directions, with equal strength.  Is that the answer you chose?

You're right that a molecule with no dipole will have the vector sum of all polar bonds cancel out.  For a molecular dipole to exist, the vector sum (from the geometric center?) must be nonzero.

[somegirl90]

Yeah C is the answer I was leaning towards and that's what I thought was the reason but wasn't sure. Thank you so much for your response   

[Dan]

Yeah C is the answer I was leaning towards and that's what I thought was the reason but wasn't sure. Thank you so much for your response   

I think you have misunderstood. C does not have a molecular dipole because:

In C, the F atoms pull electron density away from the double bond in opposite directions, with equal strength.

i.e. the dipoles DO cancel. Vector sum = 0.