Question

What determines the polarity of a molecule?

Answer 1

The vector sum of the individual bond dipoles.

Explanation:

Possibly the indication of this is the relative polarity of `"chloroform"`, `CHCl_3`, versus `"carbon tetrachloride"`, `"CCl"_4`. Both molecules have `""^(+delta)C-Cl^(delta-)` bonds, which are reasonably polar.

If we sum up the individual bond dipoles, however, `"CCl"_4` sums TO ZERO, and thus has ZERO DIPOLE MOMENT - if you don't see this consider vector addition with very symmetrical tetrahedral bond angles. Because the `1xx""^(-delta)C-H^(delta+)` and `3xx""^(+delta)C-Cl^(delta-)` bond dipoles in chloroform do not sum to ZERO, the molecule has a resultant dipole moment. `""^(+delta)H-Cl^(delta-)`, and `""^(-delta)OH_2^(delta+)` are polar molecules by this same reasoning.