Here's why:

* Perchlorate is a polyatomic ion, meaning it's a group of atoms bonded together that carries a net electrical charge.

* The chlorine atom in perchlorate has an oxidation state of +7.

* Each oxygen atom has an oxidation state of -2.

* To calculate the overall charge, we add up the oxidation states: (+7) + 4(-2) = -1.

Therefore, the valency of perchlorate is -1.