Polar Bonds, Polar Molecules: Shape Decides

0:00Carbon dioxide and water both have polar bonds. So why is only water a polar molecule?

0:07Electronegativity is how strongly an atom attracts the shared electrons in a bond.

0:13On the Pauling scale, oxygen is about 3.4, carbon 2.6 and hydrogen 2.2.

0:22Oxygen pulls the shared electrons closer to itself. Unequal sharing like this makes a polar bond.

0:29Oxygen gets a partial negative charge, delta minus, and carbon a partial positive charge, delta plus.

0:38These two charges make a dipole. Chemists draw it as an arrow from delta plus to delta minus.

0:46The other bond has the same dipole, pointing the opposite way.

0:51Carbon dioxide is linear, so the two equal dipoles cancel. With no net dipole, it is a non-polar molecule.

1:00In water, each oxygen hydrogen bond is polar too, with its dipole pointing toward oxygen.

1:07But water is bent, with a bond angle of about 104.5 degrees.

1:13Their sideways parts cancel, but their parts toward oxygen add up to a net dipole.

1:19So water is a polar molecule, with a slightly negative end and a slightly positive end.

1:26A thin stream of water bends toward a charged rod, which attracts its polar molecules.

1:32A stream of hexane, a non-polar liquid, falls straight past the rod.

1:38So polar bonds do not always mean a polar molecule. The shape decides.

1:45If the bond dipoles cancel, the molecule is non-polar. If they add up, it is polar.