Which of the floowing molecules contains polar bonds but is nonpolar?

Question

a) H2O

b) NH3

c) F2

d) CCl4

e) CH2Cl2

the answer is CCl4 but why is there a way to calculate it or what please explain, will give 5 pts to best answer

Answer 1

Yes, d) is the correct answer. The reason CCl4 is nonpolar is because the molecule is tetrahedral in shape. Each bond angle is about 109.5 degrees, so there is a symmetry about the central carbon atom. All the dipoles point outward toward the Cl atoms in a symmetrical distribution, so the dipoles cancel each other out.

Answer 2

The answer is d).  

In CCl4, there are 4 bonds arranged symmetrically around the central carbon atom.  Each bond is polar, which you can determine by comparing the electronegativity of carbon and chlorine.   The electronegativity refers to how strongly that atom attracts electrons.  When the two atoms in a covalent bond have different electronegativity, the electron pair in that bond will be shifted towards one atom, making a polar bond.

Because the 4 atoms of chlorine are arranged symmetrically, the polarity of the bonds cancel each other out, making a non-polar molecule.

In H20, the bonds are polar, and they are arranged asymmetrically, so the overall molecule is polar. (The shape of the molecule is roughly a wide V.)

In NH3 and F2, the bonds are non-polar, as well as symmetric.

In CH2Cl2, there are two polar bonds and two non-polar bonds.  Since the 4 bonds attached to the central carbon must be arranged in the tetrahedral shape, there will be two hydrogen molecules on one side and two chlorine molecules on the other side.  They can't cancel out, so the molecule is polar.

Answer 3

d)

Answer 4

Bond polarity can be calculated by looking at the difference in electronegativities between the two atoms in the bond, which can be looked up in the periodic table. Molecule polarity is determined by looking at the symmetry of the entire molecule.

H2O and NH3 both have polar bonds, and are polar molecules. F2 is a nonpolar molecule, and it does not have polar bonds because the two atoms in the bond are both fluorine (electronegativity difference is 0). CCl4 has polar bonds, and it's symmetrical around the carbon atom, so it's a nonpolar molecule. CH2Cl2 also has polar bonds, but it is a polar molecule, since it is not symmetrical.

It REALLY helps to draw them out (you can just do Lewis/electron dot diagrams) to determine symmetry. Hope that helps!