The formula for the chlorate anion is ClO3-, with the - typically written in superscript and the 3 written in subscript. The symbol Cl indicates an atom of chlorine, while the symbol O3 represents three atoms of oxygen. The minus indicates that the ion carries a charge of minus one.
The term anion refers to an ion that carries a negative charge, while an ion that carries a positive charge is called a cation. The negative charge on the chlorate anion indicates that the ion contains one more electron than it does protons. Protons contain a positive charge, while electrons have a negative charge. The charge for the chlorate ion comes from a combined plus five charge from the chlorine and a minus six charge from the oxygen.