How does molecular shape affect polarity?

1 Answer
Apr 7, 2016

Answer:

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.