# Which of the following molecules has a dipole moment? CCl4, H2S, CO2, BCl3, Cl2

Jun 25, 2014

Based on symmetry alone, we know that ${H}_{2} S$ is the only one of these molecules that has a dipole moment.

In the case of $C {l}_{2}$, the 2 atoms are identical, so no polarization of the bond is possible, and the dipole moment is zero.

In every other case except ${H}_{2} S$, the polarization of charge associated with each bond is exactly cancelled by the other bonds, resulting in no net dipole moment.

For $C {O}_{2}$, each C-O bond is polarized (with oxygen taking on a partial negative charge, and carbon a positive charge). However, $C {O}_{2}$ is a linear molecule, so the two C-O bonds are polarized in equal and opposite directions, and exactly cancel each other out. Therefore, $C {O}_{2}$ has no dipole moment.

For ${H}_{2} S$ the bonds are both polarized, but ${H}_{2} S$ is a bent molecule, not linear, so the polarizations do not cancel, and ${H}_{2} S$ has a net dipole moment.

For $B C {l}_{3}$, the geometry is an equilateral triangle of Cl atoms, with the boron atom in the center of the triangle. The polarization of the 3 B-Cl bonds exactly cancels out, so $B C {l}_{3}$ has no dipole moment.

Similarly, the 4 C-Cl bonds in CCl4 are oriented to point at the vertices of a regular tetrahedron, and they cancel each other out exactly, so CCl4 has no dipole moment.