Question

why does chlorine in chlorate ion does not have stable octet configuration ?

Answer 1

Chlorine belong to the 3^rd period of periodic table. It has vacant d orbital and it can expand its octet like SF₆, PCl₅ etc.
But in all the cases central atom is less electronegative than ligands. So that ligands can remove the electron density from central atom and hence minimise the electronic repulsion on central atom and give stable configuration.