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 = $\frac{1}{2}$ × bonding electrons For Lewis structure of $\mathrm{SO}_3$ Formal charge on S atom = 6 - 0 - $\frac{1}{2}$ × 12 = 0 Formal charge on three O atoms = 6 - 4 - $\frac{1}{2}$ × 4 = 0