Which of the following molecules has a **non-zero** dipole moment?

Net dipole moment depends on both bond polarity and molecular geometry. Three of the four molecules have polar bonds but symmetric arrangements that cancel the bond dipoles vectorially. - $\mathrm{CO_2}$ is linear (O=C=O). The two C=O dipoles point in opposite directions and cancel. Net = 0. - $\mathrm{BF_3}$ is trigonal planar with three identical B–F bonds at $120°$. Their vector sum is zero. - $\mathrm{CCl_4}$ is tetrahedral with four identical C–Cl bonds. By symmetry, net = 0. - $\mathrm{NH_3}$ is trigonal pyramidal (lone pair pushes the H atoms to one side). The N–H dipoles do not cancel, and the lone pair adds its own contribution. Net dipole $\approx 1.47$ D. The lesson: symmetry kills the dipole even when individual bonds are polar. Lone pairs and asymmetric geometry preserve it.