Got stuck on molecular structure

asdff529

The S molecule of SO2 forms a double bond with one O and dative covalent bond with another O
Meanwhile,XeO4 is a molecule which Xe forms a double bond with each O
I have a question here
Why doesnt that S forms 2 double bonds with O to extend its octet structure?
Similarly,why doesnt that Xe form 4 dative covalent bond with O to maintain its octet structure?

DrDu

Neither sulfur nor xenon extend their octet in these compounds and a distinction between double bonded and dative bonded oxygens is purely fictive.

asdff529

why do u say that "Neither sulfur nor xenon extend their octet in these compounds"
if i draw the electron diagram of Xe,i count there should be 16 outermost electrons around Xe?

DrDu

Bonding in these compounds is quite complex. Maybe you can find in your library
http://eu.wiley.com/WileyCDA/WileyTi...fContents.html

asdff529

i think im not able to understand it
btw,thx a lot

asdff529

but can i explain like this:
since electrons are indistinguishable,and they all move randomly around the atoms
so it is indistinguishable whether it is dative or pure covalent?

DrDu

Maybe this wikipedia article is helpful:

http://en.wikipedia.org/wiki/Hypervalent_molecule