Questions on polar and nonpolar molecules

[Bohrok]

Some chem students need help with homework problems similar to these later this week. and I want to make sure I've got the concepts down before I try to explain anything. This is for introductory chemistry.

1. Can a molecule have only nonpolar bonds and have a dipole?
My first thought was no, that only molecules made of the same element have nonpolar bonds. As far as I can tell, this applies to diatomic molecules like F₂ and not others like O₃ after reading up on Wikipedia.
Am I correct or is there an exception?

2. a) Would you expect CH₄ or CF₄ molecules to have dipoles?
No
b) Would you expect CH₂F₂ molecules to have dipoles?
Yes
c) Explain
Since fluorine is more electronegative than hydrogen, the "side" of the CH₂F₂ molecule with the fluorines will be slightly more negative than the "side" with hydrogens, making the molecule have dipoles.

3. Why is the polarity of the covalent bonds in NaNO₂ of little interest compared with that in a covalent molecule such as NO₂?
Not quite sure how to explain this one. The covalent bonds in NaNO₂ are in the NO₂^- ion which has a -1 charge, while the NO₂ molecule has no net charge. Is it that a net charge "overshadows" whatever polar covalent bonds may exist in something like NaNO₂?

 

[chemisttree]

Some chem students need help with homework problems similar to these later this week. and I want to make sure I've got the concepts down before I try to explain anything. This is for introductory chemistry.

1. Can a molecule have only nonpolar bonds and have a dipole?
My first thought was no, that only molecules made of the same element have nonpolar bonds. As far as I can tell, this applies to diatomic molecules like F₂ and not others like O₃ after reading up on Wikipedia.
Am I correct or is there an exception?

Think about ammonia.

 

[Bohrok]

I would think the bonds between H and N in ammonia are polar with the hydrogens being slightly positive and nitrogen slightly negative. The textbook says that electrons shared between elements that are not the same are generally not shared equally. Is there a way to tell how equally electrons are shared by two such atoms?

 

[Firelion]

Hi,
yes, with the help of the Elextronegativity:
http://en.wikipedia.org/wiki/Electronegativity
For example halogens love electrons very much while e.g Sodium gives them away more easily.

 

[chemisttree]

OK. In ammonia, is the dipole due to the polar nature of the bonds or is it due to the non-bonded electron pair?

 

[Bohrok]

Both I think: the lone pair causes a slight negative charge on the N, and nitrogen attracts electrons more than hydrogen, much like the oxygen in H₂O.
I see that it has a dipole, but not that its bonds are nonpolar.

 

[Borek]

2. a) Would you expect CH₄ or CF₄ molecules to have dipoles?
No

Not clear to me. To BE dipoles - no. To HAVE dipoles in their internal structures, dipoles that cancel out - yes. Could be that's nitpicking, but question can be easily clarified by rewording.

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