The valency of carbonate is -2. This means that it has a charge of -2.

Here's why:

* Carbon (C) has a valency of +4.

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

* In carbonate (CO₃²⁻), there are three oxygen atoms. Therefore, the total negative charge from oxygen is -6 (3 x -2 = -6).

* To balance the +4 charge from carbon, the carbonate ion needs a net charge of -2 (-6 + 4 = -2).

Therefore, the carbonate ion has a valency of -2.