Here's why:

* Chromate ion has a formula of CrO₄²⁻.

* The superscript 2- indicates that the ion has a -2 charge.

* Valency refers to the combining capacity of an element or ion, which is equal to the number of electrons an atom can gain, lose, or share to form a chemical bond.

Therefore, the chromate ion has a valency of 2 because it can form two chemical bonds with other elements or ions to achieve a stable state.