You're asking for the oxidation number of bromine (Br) in the bromate ion (BrO₃⁻). Here's how to determine it:

Rules to Remember

* Oxygen: Oxygen usually has an oxidation number of -2, except in peroxides (where it's -1) and when bonded to fluorine.

* Overall Charge: The sum of oxidation numbers in a polyatomic ion must equal the charge of the ion.

Calculation

1. Oxygen's contribution: There are three oxygen atoms, each with an oxidation number of -2, giving a total of -6.

2. Overall charge: The bromate ion has a charge of -1.

3. Bromine's oxidation number: To balance the charges, the oxidation number of bromine (Br) must be +5.

Therefore, the oxidation number of Br in BrO₃⁻ is +5.