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 $-\frac{1}{2} \times$ bonding

electrons.

For Lewis structure of $\mathrm{SO}_{3}$

Formal charge on S atom $=6-0-\frac{1}{2} \times 12=0$

Formal charge on three O atoms

$=6-4-\frac{1}{2} \times 4=0$