2023 H2 Chem P1 Q5 — Chemical bonding Solution

The answer is B (2).
Each molecule is planar about the C=C bond, and every C-F bond is polar because F is much more electronegative than C. A molecule is polar only if the C-F bond dipoles do not cancel.
• 1,1-difluoroethene, polar. Both C-F dipoles are on the same carbon, so their resultant points along the C=C axis towards the \(\text{CF}_2\) end and is not cancelled.
• cis-1,2-difluoroethene, polar. Both F atoms are on the same side of the C=C bond, so the two C-F dipoles add to give a net dipole across the molecule.
• trans-1,2-difluoroethene, non-polar. The two C-F dipoles are equal in size and point in opposite directions, so they cancel.
• tetrafluoroethene, non-polar. The four C-F dipoles are arranged symmetrically and cancel.
So 2 of the molecules are polar.
• A is wrong: 1,1-difluoroethene and cis-1,2-difluoroethene both have a net dipole, so there are two polar molecules, not one.
• C is wrong: 3 would need one of trans-1,2-difluoroethene or tetrafluoroethene to be polar, but in both molecules the C-F dipoles cancel.
• D is wrong: trans-1,2-difluoroethene and tetrafluoroethene have no net dipole, so not all four are polar.