[chem]Lewis Structure-Expanded Valence Shell

[Carolyn]

I am currently learning how to draw lewis structures and find it to be really confusing sometimes.

For example, for the molecule S03, the correct structure on the textbook is three resonance structures, each with one of the O's forming a double bond with the central atom S.

However, why can't we just put double bonds between S and all of the three O's? Then the formal charges for each atom wil be zero. Wouldn't be a more stable structure and therefore a perferable choice? Also since S is in the third period of the periodic table, so it can have expanded valence shells.

If anybody can provide some trick or method of knowing when to have expanded valence shells and when to have resonance structures, as well as how to identify other exceptions to the octet rule, I will be very grateful.

 

[Ester]

it's better that it fills its octet before making the charges be zero on a central atom. that's why there is resinance structures.