Posted by natash on Thursday, May 1, 2008 at 6:26pm.
Thanks for your answer(s). Again, it tells me where you thinking is wrong. Right on CF4. Each C-F bond is polar BUT it is tetrahedral; therefore, symmetrical and no net dipole moment for the molecule. Wrong on D. It is linear and since the electronegativity of H and N are not the same, it MUST have a net dipole moment. Therefore, C is the correct answer.
so if the electronegativities are not the same it has a net dipole? Is this only in a linear molecule?
No, it can happen in molecules that might otherwise have no net dipole. For example, CH4 has no net dipole even though the C-F bonds are polar but that is because CF4 is a tetrahedral molecule and in three dimensions the individual C-F polarities are canceled and there is no net dipole. In CH3Cl, however, there is a net dipole and the CH3Cl molecule exhibits polarity. The C-H bonds cancel in CH4 but in CHCl3 there is a C-Cl bond that doesn't cancel out with the CH3 bonds. Electronegativity is what makes the bonds polar. If the molecule is symmetric in three dimensions, there is no net dipole moment for the molecule. NH3 has polar bonds AND it has a dipole moment because it has an unshared pair of electrons (so it isn't symmetrical in three dimensions). And O=C=O has polar bonds but no net dipole because it is linear. In fact, that's how we know H2O is not H-O-H. If it were linear it would have no net dipole moment. However, we can measure the dipole moment of H2O which means it can't be a linear molecule.
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