Question

why is ClO3- a +5 in oxidation state? I need this clarified

Answer 1

Oxygen has greater electronegativity. so, less electronegative Cl
gives 5 electrons to 3 Oxygen atoms. Cl gives 5 electrons from 2p
Subshell. One electron comes outside, That's why sign (-).

Both are non-metals, so they form covalent bond, but formally,
Cl has positive oxidation number because of lower electronegativity.

In multi-atom ions, sum of oxidation numbers is same as
Charge of the ion.