Final answer:
Bromine can exhibit a +3 oxidation number in oxyanions like bromine trioxide (BrO3), where the total charge of three oxygen atoms is -6, requiring the bromine to have a +3 oxidation number to balance the compound.
Step-by-step explanation:
To determine in which substance bromine has an oxidation number of +3, we need to find a compound where bromine is combined with oxygen, as halogens like bromine (Br) can have positive oxidation numbers when combined with oxygen due to oxygen's more electronegative nature.
Halogens generally have negative oxidation numbers when they form halide compounds. For instance, in compounds such as NaBr or KBr, bromine has an oxidation number of -1. However, bromine can exhibit a +3 oxidation number in certain oxyanions or oxygen-containing compounds. An example of such a compound is bromine trioxide (BrO3), wherein bromine has an oxidation number of +3.
In order to verify this, we can use the rule that in a neutral compound, the sum of the oxidation numbers must be 0. The oxidation number of oxygen is usually -2, so in BrO3, with three oxygen atoms, the total negative charge is -6. Thus, for the compound to be neutral, bromine must have an oxidation number of +3 to balance the -6 charge from oxygen.