posted by Mary .
I am having trouble understanding polarity. I think I understand what it is - when a chemical bond occurs and there is or is not a resulting charge on the molecule. And the book says that the amount of polarity is the difference in the electronegativity between the two atoms in the bond (is my answer to the second question correct?).
What I don't understand is how to find the electronegativity, or even if the molecule has a dipole or not, if there are more than two atoms in the molecule. For instance, in the first problem, it says that CO2 is nonpolar. Why? H2O is polar, so obviously it isn't the structure of the atom that causes polarity, although our text uses the picture of children pulling on a rope in opposite directions to teach this concept. What am I missing? Thanks - this is so confusing!
First question: An individual carbon-oxygen bond is polar. Yet carbon dioxide, CO2, which has two carbon-oxygen bonds, is nonpolar. Why?
Second question: Which bond is most polar: H-N, N-C, C-O, C-C, O-H, or C-H?
The O-H bond would be most polar since it has the greatest difference in electronegativity.
last question: Yes, OH is most polar for your reason
Now on molecules, there is a big factor in the structure. If two polar bonds are aligned, the molecule is doubly polar, but if they are aligned in opposite direction, the two polar moments cancel.
Take CO2. It is a linar molecule O=C=O
each side is polarized, but in opposite directions. It has a net zero polar moment. Having a net dipole moment for a molecule means of you add up all the moments of each bond, and take consideration of their directions, then the vector (look up that word) is the net dipole for that molecule.
So in the end, two things matter:1) the difference in electronegativity of the bonded atoms (ie, how they share electrons), and 2) how the entire molecule is arranged.
Ok, having a different structure makes sense - so how would I know what the structure is like for a specific molecule?
You study Lewis structures and VSEPR theory.