A molecule's net dipole moment is the vector sum of all bond dipoles AND lone pair contributions. Even with polar bonds, symmetric molecules have zero net dipole: BF3 (trigonal planar — 3 equal vectors at 120 degrees cancel), CCl4 (tetrahedral — 4 equal vectors cancel), XeF4 (square planar — 4 F atoms symmetrically placed, lone pairs also symmetric). Molecules with lone pairs that break symmetry have non-zero dipoles: H2O (bent, mu = 1.85 D), NH3 (pyramidal, mu = 1.47 D), SO2 (bent, mu = 1.63 D). Classic comparison: NF3 (mu = 0.24 D) vs NH3 (mu = 1.47 D). In NH3, the lone pair dipole reinforces bond dipoles; in NF3, the lone pair dipole opposes bond dipoles (F is more electronegative, pulling electron density away from N).