[Byrne]

How would you determine the electronegativity difference in molecules such as CH2Cl2, NH3, or H2O?  I have to classify these molecules as either nonpolar covalent, polar covalent, or ionic and need my teacher wants us to show the electronegativity difference of a molecule before we classify it.  Atoms with two atoms are easy, but I'm not sure how to determine the electronegativity difference for molecules like the ones listed above.

[Mitch]

I have never heard of molecules being considered with electronegativities. Its typically only associated with atomic phenomena.

[Byrne]

I have never heard of molecules being considered with electronegativities. Its typically only associated with atomic phenomena.

Sorry, I meant considering electronegativities of the atoms within those molecules I listed.

[Mitch]

Its called polar for molecules. And molecules with a net dipole moment will be polar.

[plu]

Draw out the geometric structures of the molecules and look at each of the bonds in turn.  I remember the way I did it in high school was to look at the differences in the electronegativity values of each of the atoms to determine if each of the bonds were ionic, polar covalent, or non-polar covalent depending on the magnitude of the differences.  Your teacher might want you to do it some other way but whatever method you use, you will always be looking at (as mitch said) the net dipole moment of the molecular.  That is, if electron tend to stay on one side of the molecular versus another.