# Based on formal charges, what is the best Lewis structure for ClO_3^-?

${\left(O =\right)}_{2} C l - {O}^{-}$; there is a formal lone pair on the chlorine atom.
There are $7 + 3 \times 6 + 1 = 26 \text{ valence electrons}$ to distribute over 4 centres. And given 13 electron pairs, the Lewis structure as given is reasonable. The chlorine atom bears a formal lone pair.