Mercury(I) nitrate is a bit tricky because mercury(I) exists as a diatomic cation, Hg₂²⁺.

Here's the breakdown:

* Mercury(I) ion: Hg₂²⁺

* Nitrate ion: NO₃⁻

To balance the charges, we need two nitrate ions for every mercury(I) ion.

Therefore, the formula for mercury(I) nitrate is Hg₂(NO₃)₂.