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.

http://www.chem.purdue.edu/gchelp/vsepr/

Understanding polarity can indeed be confusing, but let me break it down for you. Polarity in molecules is a result of the difference in electronegativity between the atoms involved in a chemical bond. Electronegativity is a measure of an atom's ability to attract electrons towards itself in a chemical bond.

To determine the electronegativity, you can refer to a table of electronegativities, such as the Pauling scale. This scale assigns values to each element, representing their electronegativity. The larger the electronegativity value, the stronger an atom attracts electrons towards itself.

In the case of CO2, where you have two carbon-oxygen bonds, the overall polarity of the molecule is determined by the geometry and symmetry of the molecule. Even though individual carbon-oxygen bonds in CO2 are polar due to the difference in electronegativity between carbon and oxygen, the molecule as a whole is nonpolar. This is because the two polar carbon-oxygen bonds are symmetrical and located on opposite sides of the carbon atom. As a result, the bond polarities cancel each other out, resulting in a nonpolar molecule.

On the other hand, in the case of H2O, the molecule is polar. The oxygen atom has a higher electronegativity compared to hydrogen, causing the oxygen atom to attract the shared electrons in the O-H bonds more strongly towards itself. This creates a permanent partial negative charge (δ-) on the oxygen atom and partial positive charges (δ+) on the hydrogen atoms. The molecule itself has bent geometry, causing an asymmetry in the distribution of charge, resulting in a polar molecule.

To determine which bond is most polar among H-N, N-C, C-O, C-C, O-H, and C-H, you can compare the electronegativities of the atoms involved. The greater the electronegativity difference between the two atoms, the more polar the bond will be. In this case, you correctly identified the O-H bond as the most polar since oxygen has a higher electronegativity compared to hydrogen, resulting in a greater difference in electronegativity between the two atoms.

To summarize, when determining polarity in molecules with multiple atoms, you need to consider both the electronegativities of the atoms involved and the overall molecular geometry and symmetry.