[shaggybill]

In my textbook, they have this:



I don't understand why the top set of molecules are shaped differently than the one at the bottom of the picture. My book (so far) says absolutely nothing about why these two molecules have different shapes. Why does one have its atoms at an angle, whereas the other has them straight across?

I see that in the next chapter they talk about molecular geometry. Is the idea that I am supposed to take their word for it for now, and will learn about why later?

The reason for my question is because I don't want to do these problems wrong when I work on them. Can I draw the top molecule like the bottom molecule and still be right (not including the bonds, of course)?

[azmanam]

For starters, they are different compounds.  The three resonance structures on top correspond to sulfur trioxide (SO₃)  According to VESPR theory, this molecule is trigonal planar.  Note that there is no lone pair on sulfur, and your book indicates the compound has 24 valence electrons.

The bottom structure is the sulfite anion (SO₃^2-).  It has 26 valence electrons - with a lone pair on sulfur.  According to VESPR theory,  the sulfite anion is trigonal pyramidal.  If you count the lone pair of electrons, the four 'groups' (three oxygen atoms and a lone pair) point to the corners of a tetrahedron, similar to ammonia.

sulfur trioxide - http://en.wikipedia.org/wiki/SO3
sulfite anion - http://en.wikipedia.org/wiki/Sulfite
ammonia - http://en.wikipedia.org/wiki/Ammonia
VESPR - http://en.wikipedia.org/wiki/Vespr

[azmanam]

I think the point the book was trying to make is that with 24 valence electrons on a compound with the empirical formula SO₃, you can draw 3 resonance structures.  With 26 valence electrons on a compound with the empirical formula SO₃, you can only draw 1 resonance structure.

At this point in the text, I don't think the book is concerned with molecular shape or geometry, just valid Lewis dot structures and resonance forms.

[shaggybill]

Great, thanks for the reply. I thought maybe I was missing something. At this point the book has not got into VESPR, so I think you're right. I should not be concerned about the geometry yet.