What is the molecular formula for chlorate ion?

It isn't actually the 'molecular' formula because it is not molecule you ask about, but here is the formula of the chlorate anion:

ClO3-

in which the oxidation state of Cl is +5 and of O it is -2 (as ever in oxy-compounds).

(The acid of this base is HCLO3, name: hydrogen chlorate, or better known as chloric acid).