A molecule has a non-zero dipole moment when there is a separation of charge due to differences in electronegativity between the bonded atoms and the molecule's geometry does not lead to cancellation of these individual bond dipoles.
CCl4: Carbon tetrachloride has a tetrahedral geometry. Although the C-Cl bonds are polar, the symmetrical arrangement of the four chlorine atoms around the central carbon atom leads to the cancellation of the bond dipoles, resulting in a net zero dipole moment.
HI: Hydrogen iodide is a diatomic molecule. Iodine is more electronegative than hydro- gen, creating a polar bond and a net dipole moment pointing towards the iodine atom. Because it's diatomic, there are no other bond dipoles to cancel it out.
CO2: Carbon dioxide has a linear geometry. The C=O bonds are polar, but the molecule's symmetry results in the cancellation of the bond dipoles, leading to a zero net dipole moment.
BF3: Boron trifluoride has a trigonal planar geometry. The B-F bonds are polar, but their symmetrical arrangement around the central boron atom results in the cancellation of the bond dipoles, leading to a net zero dipole moment.