Polarity

Deciding the Polarity of the bonds and the Polarity of the molecule is very simple.

A bond is Polar if there is not an even distribution of electrons between the atoms. A bond is Non-Polar if the electrons between the atoms are evenly distributed. For example HCl is a Polar molecule because the bond is more negative on the side of Cl, where as H2 is Non-Polar because the electrons are balanced throughout the bond.

It is very similar to find the Polarity for a molecule.  For a molecule one must look at all ends of the molecule and if there is any end that is more negative than the rest of the molecule then the molecule is Polar; if the electrons are spread out evenly throughout the entire molecule then it is Non-Polar.

Acetaldehyde is a Polar molecule because the right side of the molecule is more negative than the left. The reason for this is because there are unbonded electrons on the O atom.