Which molecules are polar? For each molecule that is polar, specify the direction of its dipole moment.

(a) CH₂Cl₂

A molecular dipole moment is determined as the vector sum of the bond dipoles in three-dimensional space. Thus, by superimposing the bond dipoles on a three-dimensional drawing, the molecular dipole moment can be determined. Note that on the following diagrams, the dipole moments from the C- H bonds are ignored because they are small.

(b) HCN

(c) H₂O₂
The H₂ O₂ molecule can rotate around the O-O single bond, so we must consider the molecular dipole moments in the various possible conformations. Conformations such as the one on the left have a net molecular dipole moment, while conformations such as the one the right below do not. The presence of at least some conformations (such as that on the left) that have a molecular dipole moment means that the entire molecule must have an overall dipole moment, in this case  = 2.2 D.