VITEEE 2012 Paper

Formal charges help in selection of the lowest energy structure from a number of possible Lewis structures for a given species. Generally the lowest energy structure is the one with the smallest formal charges on the atoms.
Formal charge on an atom = total no. of valence electrons-non -bonding electrons =

1

2

× bonding electrons
For Lewis structure of SO3

Formal charge on S atom
= 6 - 0 -

1

2

× 12 = 0
Formal charge on three O atoms
= 6 - 4 -

1

2

× 4 = 0