Why is the oxidation number of Br in BrO3 5?

Answer 1

BrO3 is an anion of minus one (-1) charge.

BrO3^-1

Using oxygen as the standard at '-2'

Then the oxygen moiety is 3 X -2 = -6

So doing a 'little' sum

-6 + Br = -1

Add '6' to both sides

Br = 5 (The oxidation number of Br).

NB Using '-2' for oxygen is a good standard for most molecules/ions. However, oxygen's oxidation number does vary , but not in very many molecules/ions.

Answer 2

In the bromate anion, BrO3- Br has the oxidation number of 5. To work this out remove each O from the ion as O2- this leaves Br with a notional charge of +5