Why bond polarity and molecular geometry determine molecular polarity?

1 Answer
anor277
Jan 24, 2016

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.

Related questions
See all questions in Polarity of Molecules
Impact of this question
8631 views around the world
You can reuse this answer
Creative Commons License