Question

What is the formal charge on the `Br` and `O` atoms in the `BrO_3^-` ion?

Answer 1

`2xx` `Br=O` bonds, and `1` `xx` `Br-O^-` bond. One `O` atom bears a negative charge, and the remaining atoms are neutral.

Explanation:

The conventional Lewis structure of bromate ion is `(O=)_2Br-O^-`. Around the doubly bound oxygens there are 6 electrons (hence `O` is neutral); around bromine there are 7 electrons (4 from the doubly bound oxygens, 1 from the singly bound oxygen and 2 from the lone pair); and around the charged oxygen, there are (necessarily) 7 electrons (one from the `Br-O` bond and 6 lone pair electrons), so this centre (formally) has a negative charge.

Note that the number of electrons around the atom of interest determines the formal charge of that atom; single bonds share the electrons between the bound atoms, lone pairs devolve solely to the atom concerned.

Of course in the actual molecule, all of the oxygen atoms are equivalent.