You're likely referring to the 7 oxygen atoms in ammonium dichromate, which has the chemical formula (NH₄)₂Cr₂O₇. Let's break down why:

* Dichromate ion (Cr₂O₇²⁻): The dichromate ion contains 7 oxygen atoms.

* Ammonium ion (NH₄⁺): Each ammonium ion contains no oxygen atoms.

Since ammonium dichromate has two ammonium ions for every dichromate ion, the total number of oxygen atoms in the compound remains 7.

Let me know if you have any other questions!