Question

How does molecular shape affect polarity?

Answer 1

The polarity of a molecule depends on the geometric sum of the individual dipoles.

Explanation:

Let's consider two molecules: `CX_4 ("X =Cl")`, and `CHCl_3`.

Now carbon tetrachloride is a non-polar molecule; there is no charge separation, even though the `C-Cl` bonds have some degree of polarity, i.e. some charge separation in the `C-Cl` bonds. On the other hand `CHCl_3`, chloroform, is reasonably polar. Why?

Molecular polarity is the vector sum of the individual `C-Cl` bond dipoles. Because carbon tetrachloride has a very symmetric tetrahedral geometry, the bond dipoles vector sum to zero. When we perform the same operation for chloroform, the `C-Cl` and (weak) `C-H` dipoles do not sum to zero, and there is a resultant vector, and a resultant molecular polarity.