How to know if this is the best lewis dot structure regarding the exception of octet rule

Question

Some examples seem to have More Suitable lewis structure although they already achieved stable electron arrangement in another structure,how could that be?and how formal charges help to determine the best structure of an exception compounds of octet rule

Answer 1

Yes, it is true that certain elements may violate the "octet rule", and this is mainly because they are able to expand their octet into d orbitals of the 3rd energy level (or higher). Common examples would be Si, P, S and Cl.  Formal charges help to determine the "best" or most stable Lewis structure by finding the Lewis structure with the lowest formal charges on each atom.  Of course, they will add up to the net charge on the molecule, but a structure with +2 and -2 (for a net charge of zero) is not as "good" as a structure with 0 and 0, for example. 

 

A good example of this is SO₃ (no charge, just sulfur trioxide). If you draw it with S at the center, and single bonds to each O, you have S with a formal charge of 3+ and each O has a formal charge of 1-.  If you have S at the center with double bonds to each O (and 2 lone pairs on each O), then S has an expanded octet BUT the formal charge on S is now zero, and that on each O is also zero.  This latter structure is more stable and hence, the preferred one.