The oxidation number of this molecule, called a bromate molecule, is    -1.  It should be correctly written BrO3-.  It has this net charge of negative 1 because the bromine has an oxidation number of +5, while oxygen has it's normal oxidation of -2.  3 oxygen atoms would give an overall valence electron vacancy of -6 electrons, which accepts bromine's 5 electrons to form a covalent bond with an overall oxidation number of -1.  Bromine is an element found in group 17, the halogens.  The halogens normally have room for one electron to complete their outer shell.  Other notable halogen family members are fluorine, chlorine, iodine, and astatine.  The word halogen literally means "to form salt".  The combination of a halogen with any of the metals found on the metallic side of the periodic table forms a salt of some type.