We can draw three inequivalent Lewis structures for the sulfite ion , SO32- . The concepts of formal charge and electronegativity can help us choose the structure that is the best representation. 1. Assign formal charges to the elements in each of the structures below. Note: Count oxygen atoms starting from the left for each structure. A B C Formal Charge S O1 O2 O3 2. The best Lewis structure for SO32- is

The formal charge of an atom in a molecule is the assumed charge the atom would have if the electrons in the bonds were evenly distributed between the atoms.

Formal charge can be calculated as follows:

Formal charge = number of valence electrons - ( number of non-bonding electrons + ½ bonding electrons)

Formal charges are used to determine the best possible Lewis structure of a molecule or ion. The following rules are applied:

1. A structure in which all formal charges are zero is preferable to one in which some formal charges are not zero.

2. If non-zero formal charges must exist in the structure , the arrangement with the smallest non-zero formal charges is most preferred.

3. Lewis structures are preferable when adjacent formal charges are zero or of the opposite sign.

4. The structure with the negative formal charges on the more electronegative atoms is preferred among other structures of the molecule or ion.

From the above rules, structure B has negative charges on the more electronegative oxygen atoms and has more zero formal charges on the atoms. The central atom, sulfur also has a zero formal charge.