Well, basically, all the question is asking for is, which molecule has a resonance structure. I just memorized these in AP Chemistry; you have to know the few basic molecules that commonly occur in problems, such as O3, NO3, and SO3.
The answer is C. SO3 I'm assuming that you know how to draw Lewis dot structures, if you don't then you might have to re-read the chapter on this...
If you haven't... here's how below:
Think about it, sulfur has 6 valence electrons, oxygen has 6 as well, so SO3 has 6*4=24 electrons. Give each atom an octect first, you will notice there can be 32 electrons when each octect is filled. However, since you only have 24 electrons to work with, your only option is to make double bonds. So take away a pair from two adjacent atoms and make them share the bond. As you do this, you will end up with a double bonded S=O and two single bonded S-O. With that, you now have 24 electrons, which is what the molecule should have. SO3 is resonant because that double bond could be on the other two S-O bonds (Here is a link to a visual representation:
Another way of doing this is by elimination. You should know that N2 is triple bonded together. And H2CO does not exist, I think you meant H2CO3, which is just a normal acid molecule. H2S is exactly shaped like H2O, except that the S replaces the O, it is not resonant. CHCl3 is a tetrahedral shaped molecule, it is just a normal molecule. BCl does not exist, either, I think you meant BrCl, which is just a normal molecule, too.
If you need any more help, do not hesitate to ask me! =)
2009-06-18 04:49:21 補充：
The website works perfectly fine for me.....
H2O has 2 lone pairs, its two other electons bond with the single electrons of Hydrogen.
Here's a picture of H2O
2009-06-18 04:52:51 補充：
I found another picture of SO3, I hope this one works.
2009-06-18 04:53:45 補充：
I've responded. :)
2009-06-18 04:55:51 補充：
Actually, check the link again (the first one)
DO NOT COPY THE PARENTHESIS TOO. I think that was the reason as to why you couldn't see it.
Here it is again, it's a better picture...
自己, Took AP Chemistry in the United States, myself, myself