Here's why:

* Ammonium (NH₄⁺) is a positively charged ion (cation).

* Chromate (CrO₄²⁻) is a negatively charged ion (anion).

To form a neutral compound, the charges must balance. Since ammonium has a +1 charge and chromate has a -2 charge, we need two ammonium ions for every one chromate ion.

Therefore, the correct formula for ammonium chromate is (NH₄)₂CrO₄.