There is no ammonium in the chemical formula for dichromate. Dichromate refers to the dichromate ion, which has the formula Cr₂O₇²⁻.

Ammonium, on the other hand, is a cation with the formula NH₄⁺.

Dichromate and ammonium are separate ions and would only appear together in a compound like ammonium dichromate, which has the formula (NH₄)₂Cr₂O₇. In this case, the subscript for ammonium is 2.