Here's why:

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

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

* In the silicate ion (SiO₄⁴⁻), there are four oxygen atoms, each with a -2 charge, totaling -8.

* To balance the charges, the silicon atom contributes a +4 charge.

* Therefore, the overall charge of the silicate ion is -4.