# How does molecular shape affect polarity?

Apr 7, 2016

Let's consider two molecules: $C {X}_{4} \left(\text{X =Cl}\right)$, and $C H C {l}_{3}$.
Now carbon tetrachloride is a non-polar molecule; there is no charge separation, even though the $C - C l$ bonds have some degree of polarity, i.e. some charge separation in the $C - C l$ bonds. On the other hand $C H C {l}_{3}$, chloroform, is reasonably polar. Why?
Molecular polarity is the vector sum of the individual $C - C l$ 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 $C - C l$ and (weak) $C - H$ dipoles do not sum to zero, and there is a resultant vector, and a resultant molecular polarity.