Here's why:

* Silicon (Si) has a valency of +4.

* Oxygen (O) has a valency of -2.

* In the silicate radical, there are four oxygen atoms, giving a total negative valency of -8.

* To balance the valency of silicon, the silicate radical needs to carry a charge of -4.

Therefore, the overall valency of the silicate radical (SiO₄)⁴⁻ is -4.