Question

Why bond polarity and molecular geometry determine molecular polarity?

Answer 1

Because molecular polarity results from the VECTOR sum of the individual bond dipoles. Vectors have magnitude and direction, so polarity is in part a function of geometry.

Explanation:

Take carbon tetrafluoride, `CF_4`; the `C-F` bonds all show charge separation in that fluorine polarizes electron density towards itself (i.e. a `C-F` bond is polar!). Nevertheless, if we take the vector sum of these individual bond dipoles, the indvidual vectors sum to zero.

The same reasoning applies to `"C"Cl_4`, carbon tetrachloride, and `CHCl_3`, chloroform. Carbon tetrachloride is a non-polar solvent in that the bond dipoles will sum to zero (they must, because of the molecular symmetry, tetrahedral with `109.5` `""^@` bond angles. On the other hand, chloroform has a degree of molecular polarity and is thus a polar solvent, inasmuch as the individual bond dipoles DO NOT sum to zero, and there is a resultant molecular polarity.

So how to conclude? Two factors determine molecular polarity: (i) bond polarity (which can be assessed by the electronegativity difference between the bound atoms); and (ii) the VECTOR sum of the bond dipoles.